Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.  

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Even killing them... Killing them could be an option, as long as it was a last resort, that is what Undyne had said, right? It was something that kept repeating over and over inside Frisk's head, so much that she wasn't even thinking about what she was thinking about. Not until the actual mission, where they were actually there: her, Gaster, Blair, Asriel, and Grillby, and the skeleton monster had questioned if she was okay while they were making their way towards the location on foot, deciding a car might get easily noticed. Frisk replied, saying that she was fine, but truthfully, she felt somewhat guilty that she was so casually thinking of what the death of two people might possibly be like.

It wasn't every day that her brain decided to think such thoughts. Normally, she was the one who was trying to steer others away from the thought of murder, wanting to suggest that there had to be some kind of better way. She was thinking all of this just as Blair was muttering to Grillby how it might be best for him to go around the back with Asriel, seeing as he was made of flame so if he was at the front of the house, he might get easily spotted. "Frisk." Blair said, giving the human girl a slight nudge, snapping her out of it. "Focus. Look, if you want my honest opinion, if you're thinking of killing them, just do it." 

Of course the fox would say such a thing. Like anyone who had been a part of the Mafia or some kind of dangerous group, killing wouldn't be a regret for them; they likely felt like they were doing the world a favour. As Sans had said: they were wastes of oxygen, people that the world would be much better off without. Had these people not existed, her parents never would have died. Had someone got rid of them earlier, she would have still had her mom and dad. The weird thing was, if her parents never died, she probably would have never met Sans, because she then never would have been on the street.

But her mind didn't fucking care. All she wanted was to make those men pay for what they did to her mother and father. It was probably the first time in awhile where she had felt real genuine anger. What if these men somehow showed up one day and tried to cause harm to her or someone she loved again? What if they showed up and ended up harming her and Sans' baby? They were a total threat, and if kept alive, they might just hurt someone else. Right now, her mind really was pushing her to kill them. 

Reaching the front of the house, while Grillby and Asriel went around the back, the trio (Frisk, Blair and Gaster) stuck to the shadows, while moving in on towards the  house. The fox was the first to try the door, only to be shocked when the door wasn't actually locked. "Be careful." Gaster whispered. If the house was abandoned, chances were that it was pretty dark inside as they likely wouldn't have any power, and with it growing further into the night, they would need light to see. Good thing they had Grillby then. 

"Alright, Frisk, you hang back for the moment, I'll start moving in with Gaster, call for us if you hear or see anything." She said, just seeing Frisk nod with a frown plastered across her face. The fox sighed, wondering if it was a good idea to bring Frisk with them. After all, the girl had appeared quite miserable ever since leaving the CIA headquarters. She couldn't say that she blamed her, but at the same time if she was being held back by her thoughts, it was problematic. 

When venturing into the dark, Blair trudged carefully, with Gaster seeming to do the same; he was also being cautious. Luckily, despite it being dark, they could still just see each other, so Gaster was signaling with his hand for her to check out one side of the house, while he would check out the other. The fox nodded, splitting ways from the skeleton, as she continued sneaking almost silently. When making her way towards the kitchen, she spotted the back door slowly opening, and that's when she spotted Grillby and Asriel who gave her a small wave. 

"You should stay down here with Gaster." Blair whispered to Grillby, considering he was made of flame, so he might be seen if he tried to go upstairs. And by the looks of it, downstairs appeared pretty silent, so there didn't seem to be anyone down there. Chances were that these two people might have even been asleep, and with no sirens to alert them that the police were coming, it's not like they would be woken up, right? "Me and Asriel will head up." 

"Wait, where's Frisk?" Asriel questioned, only for Blair to explain that she left her to keep watch at the door. It's not that she didn't want to trust Frisk with the task, but... Though, as she was thinking that, she felt someone brush past her, and when she turned to look, she saw that it was actually... 

"Frisk!" Blair hissed after the Dreemurr who was already making her way up the stairs. Both Blair and Asriel followed after her, trying their best to be quiet, while Frisk already appeared ready to rush in. "Wait, quietly." Blair told the brunette haired girl who already appeared ready to grab the handle of one of the bedroom door. Frisk simply nodded, while holding a knife in her hand. Cautiously, she pushed it open, only for her eyes to make out two figures in the dark, lying down on separate beds. Using the same tactic Gaster had, Blair signaled for Frisk to try and take out one of them, while they would deal with the other. It wasn't like Undyne was here, so killing them didn't seem like a bad idea. Besides, trying to arrest them appeared to be a more dangerous idea with them sleeping on their backs. 

Holding tightly onto the knife in her hand, Frisk narrowed her eyes, as her heart really began to hammer rapidly in her ears, while she rose the knife she had up into the air, seeming ready to make a strike. This feeling, this adrenaline rush was similar to when she had to take down Andrew, and all she could hear was the blood roaring in her ears as she did what she had to do. It wasn't like she hadn't killed someone before, so why should this be any different. This was to save future lives. This was for her parents. 

"Don't move." A voice said, after Blair had yelped out in shock. Frisk spun around in bewilderment, only to see one of the men grappling Blair into a hold, whilst holding a gun to her head. Asriel was standing still, knowing he could probably attack this stranger quite quickly, but would he be quick enough to stop the guy from pulling the trigger? "If you make a move, I'll blow this bitch's brains out. Don't think I won't." They threatened, right as Frisk suddenly felt someone cover her mouth and grab a hold of her. 

"Drop it." The other man said, twisting her wrist so that she dropped the knife. "I know others are here! And I would stay downstairs if I were you, unless you want to come up to your friend's mangled corpses!" He yelled, while Frisk stood with her eyes wide, feeling terror run through her entire body. These were the men that had murdered her parents, she just knew they were; now they had captured her. Was she going to die in the same way that they had? Were they just going to kill her, Asriel and Blair with no questions asked? 

Once again, was she just going to be a failure who would get captured and she would have to expect someone to rescue her? Any time she had gotten into trouble, someone had always been there to help her. She remembered that time when she had been all alone because Sans had left her in anger, and she had been forced to try and find her way back to his apartment all alone, and then she was ambushed by a couple of strange men. It wasn't until Sans showed up and saved her that she was safe. He looked out for her countless times after that. And he hadn't been the only one. 

Would she die here? 

She couldn't die here. She promised. She had a child to return to, a man who loved her, who she adored so dearly in return. He would never forgive her if she didn't come back, and that is what he had said. Part of him thought he was kidding, but she knew he wouldn't say such a thing unless he was at least somewhat serious. No. No way was she going to get taken down here. Looking towards Blair, her eyes connected with the fox's, who seemed to be giving her a look that they should try to take these bastards down. And Frisk couldn't agree more.

At that moment, Frisk managed to get her mouth free, and using her mouth, she decided to bite down onto the man's thumb, not caring whatsoever if she drew blood. The point was to try and get free after all, so even as the man cried out in pain, she just continued to bite, hard. That's when the man let go of her, shoving her down to the floor. "Bitch! She fucking bit me!" He snapped. Around the same time, Blair had gone for the gun, trying to knock it out of the other man's hand, and upon seeing she might need help, Asriel went to tackle the man down, so he wouldn't try to shoot her. 

"I bet you're really hoping for a death wish, aren't you!" The man who had attacked Frisk decided to tower over her when she was on the ground, and went to wrap his hands around her throat, forcing her down to the ground. Blair and Asriel were currently still busy trying to fight the gun off of the other figure to really help her. "I'll kill you!" He yelled at her, while she gasped out for air and tried to struggle against his hands around her throat. But that's when she remembered: knife. One of her arms went to reach out of it, hoping desperately to find it, while he was too busy trying to strangle her to death. Come on... Come on... She felt her finger tips just barely brush against it, before finally she managed to pull the knife into her grasp.

"This... Is for my p-parents!" Frisk yelled out, first going to stab the man in the shoulder with the knife, hearing him screech out in agony from feeling the metal pierce his skin, but she wasn't even done there. She was so caught up in the moment of getting her revenge, that she pulled the knife out and managed to catch him in the side of the neck. It was then that he let go of her and she was able to overpower him. When he was down on the ground, she just kept using the knife, attacking him over and over, while screaming and yelling at him. Gaster and Grillby ran up the stairs, seeing this happen, while Asriel and Blair had managed to pin down the other man, as they were sitting upon his back. 

Grillby went over to Blair and Asriel to put the man's hands behind his back to cuff him, as Undyne had actually given them handcuffs to do this, while Gaster headed towards Frisk, who was still busy acting out on her rage. "Hey, Frisk! Stop, it's okay, it's over now." Gaster told her, going to try and pull her back, but the moment he tried she just seemed to cry and freak out a little. "Frisk. Look at me, it's Gaster. You're okay. See?" Gaster said, angling Frisk's face to look right at him with his own hands. Tears trickled down Frisk's face as she stared back at the other skeleton, and that's when she slowly started to come to. She dropped the knife, really starting to sob. 

"I... I k-killed him..." Frisk stammered out, only to be pulled into a reassuring hug by the fatherly skeleton. At the time, murdering him had felt like a good idea, but now that she had come back from her moment of rage, she was just questioning what she had done. That was another person she had killed now, and this time it had been purely in cold blood. She didn't know whether to feel disgusted with herself, or proud of herself for taking care of someone who was an absolute disgrace to this world. 

"I know, Frisk... But he can't hurt anyone now." Gaster said. "And we've at least got this guy to bring back. He'll be able to rot in a prison cell and think about what he's done." The skeleton went on. 

"Fuck you." The guy spat, being harshly pulled off of the ground by the goat who looked at the human man with his own look of displeasure and disgust. It was one of those rare times you saw Asriel looking more serious and it looked as if getting on his bad side would be a bad idea because otherwise he might just kill you with that stare alone. It was a little scary when you were more used to a calmer version of Asriel. 

"Let's just take this guy and get the hell out of here..." Blair exhaled, as Frisk and Gaster went to get off of the floor. "Don't beat yourself up about it, Frisk. He would have killed you had you not killed him. And it isn't like anyone is going to miss him anyway." The fox said, going to put an arm across the other girl's shoulders as they walked together, going to leave the house as a group, leaving the man's bloodied body behind. Undyne could simply deal with that. 

"I guess so..." Was all Frisk quietly said, as it would likely take her a moment to get over what she had just done. She did say that she wanted to get them back for murdering her parents, and she had thought to herself that it would be better off to kill them, knowing it might just keep her future family safe. Those two had started off as two robbers who had panicked when killing her parents, and now they just looked like shitty people who were fine with killing anyone who got in their way.

Maybe killing that guy wasn't the worst thing in the world after all.