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

------------------------------

"Okay... What the fuck just happened?!" The fox in the group was the first to complain, as she was just standing with her hands on her knees, as the group were all going to catch their breath. No one was really sure how long they had been running for, but it had led them to climbing up onto roofs, jumping across from one building to another. Blair had forgotten how crazy it could sometimes be when you were around that damned skeleton. Or just being in the Mafia in general, sometimes the most bizarre or exhilarating of things happened; even in her other group it hadn't been this insane. Now, they were still standing on a rooftop that they had gotten to, and it did look like it had a staircase going back down into the building, as there was a door on the rooftop. But going down probably wasn't for the best, as Undyne's crew were after them still, and would likely be coming up. 

"is everyone okay?" Sans questioned, ignoring Blair as it seemed like thankfully no one had been severely injured.  He glanced towards his brother, who appeared to be fussing with the door. "hey, what are ya doin', papyrus?" He questioned going to walk over to his side, only to see that Papyrus had created his own door jam. He had used his magic to create a bone, and propped it up against the bottom of the handle to the ground. It was one of those handles you had to force downwards to open the door, so that would cause some difficulty if they were found. "hey, that's smart."

"THANKS! I WAS THINKING HOW WE NEEDED SOME TIME TO REST, ESPECIALLY YOU." Papyrus beamed at his older brother who rose his eyebrows a tiny bit. "YOU'RE NOT JUST LOOKING AFTER YOURSELF ANYMORE AFTER ALL."  He pointed out, which Sans sheepishly smiled at as his pupils trailed towards the floor once again. That was true, what Papyrus was saying. If anyone did need to be extra careful, it was him. But he was just so concerned about someone getting hurt for him when they didn't need to. Quite frankly, he probably wouldn't care as much if it was someone like Andrew or Blair that got hurt, but if it was someone like Frisk or Papyrus... His soul would just hate to put up with the burden. 

"Well, at least we're out of the way of everyone now, and I'm sure it'll take some time for those people to find us, so we can continue what we were fucking talking about because you decided to run away." Blair complained from where she was standing and Sans turned around to face her, staring at her with an expression of annoyance. It was no surprise that her whining got everyone in the group looking at her. Even Frisk was furrowing her brow, seeming ready to tell the fox to shut up.

"Blair, just stop it..." Andrew murmured. "It's not worth it."

"he's right." Sans said to Andrew's surprise. "all ya are doin' is tryna pick a fight. and truthfully, i'm done with fightin' with ya. ya should feel lucky i haven't killed ya yet. but do feel free to give me a fuckin' reason to throw ya off this damn roof." Sans spat, knowing that if he had to, he might not just hesitate to do as he said. It was getting close to the point where he wouldn't care what anyone else would think. It wasn't like she was an innocent person after all, and the only person that really cared about her was that damn human, Andrew. He knew if he killed her, the guy would probably never let him hear the end of it. That, or someone else here would disapprove that he broke up the couple. But he didn't know how true that second option would be. 

"Give you a reason? I'll give you a fucking reason." Blair started, at least until they heard banging suddenly coming from the door that Papyrus had made difficult to open. And that's when they knew that they were out of time, and they should probably get a move on. Rolling his eyes, Sans simply shoved past the fox, only to decide with the others what their next course of action would be. They could simply jump to the next roof and then start running again. 

"alright, asriel, ya go first." He trusted the goat to make the jump, then he could simply help whoever went next and so on and so forth. So, with a nod, the tallest goat went to take several steps back, before going to make the leap to the next roof. He just made it, and went to pull himself up with ease, landing him on his own two feet. "chara, your turn." Sans said, allowing Chara to go next, only to allow her to repeat the same action as Asriel and she was caught by the goat and he helped pull her up. They did this with Papyrus, then Andrew. Next was Frisk. 

"Sans, you should go next." Frisk told her boyfriend, only for him to shake his head. He only trusted her to go next because he wanted the others to catch her and make sure she didn't fall. That, and he didn't trust her on this roof alone with Blair, so he wanted her to go next. It took some debating back and forth, until the human girl gave in and sighed. Truthfully, she found it difficult not to be terrified for this jump, as any jump they had taken between buildings had been pretty scary. But she was lucky just like any other time to make it. "Alright, now come on!"

"Now, hold on a fucking second." The fox yelled from over his shoulder. "I'm sick with being interrupted. Let's just deal with this right now and get it over and done with." She said. The skeleton growled, turning to face her as she was walking towards him. It was then at that moment that he went and grabbed her by the collar of her shirt, actually going to force her off the edge of the roof, so she was just dangling by her shirt. Andrew was watching from where he was standing, horrified.

"get what over and done with?! is now really the fuckin' time? or are ya just tryin' to continue with your whole shit of wantin' to ruin my life? 'cause if ya wanna do that, do it later." He snapped. "or actually, no, ya know what? you're right. let's just get this done, now." He finished, and it was at that moment that he just let go of her, wanting to allow her to fall to her death. Instead of doing that, she managed to grip onto the end of the roof, but the skeleton was already leaving her, getting ready to make his leap to the other roof that the rest of the group were already on. Andrew yelled.

"Sans! Don't just leave her!" He shouted, irking Sans like crazy. Was that idiot fucking serious? Any minute that door behind him was going to get smashed in and a large number of FBI agents would likely come pouring out of it, and he was asking him to risk that and go back towards Blair to help her up, as it appeared the fox wasn't really managing very well to pull herself up. "Please! I love her, so don't just leave her! I know she's difficult but... What if someone who didn't like Frisk tried to leave her behind, would you let them?!" Andrew went on desperately, hoping Sans would listen to him. 

Fucking... Those two always had to be such an annoyance it was unbelievable. If he had the option, he likely would have killed him as well, but as per usual, Frisk wouldn't approve. He glanced over his shoulder towards where Blair was dangling, and to tell the truth he was very sure he was just going to leave her to fall. Why should he care about how Andrew felt when he hated him anyway? But then it was true, if someone had left his love to die - Frisk - he would be utterly torn up. Broken. He would be lost without her. But this was Andrew's love, not his... Which was why he couldn't believe he was doing this. 

"Fuck!" Blair was ready to scream and close her eyes tightly as she had lost her grip off of the edge of the building she was holding onto. But just as she was ready to fall, she felt a hand wrap around her wrist that stopped her from falling. When opening her eyes and looking up, she found it hard not to allow her mouth to fall agape as she saw that Sans had actually come back to save her. "...Why?"

"since ya like stallin' so much, give me a good fuckin' reason why i shouldn't drop ya. tell me why ya did it." Sans demanded, seeing Blair look at him in bewilderment for a second. "tell me why the fuck ya suddenly decided one day that ya were better off without me, why ya decided to betray me! if ya can't give me a reason, then kiss your ass goodbye!" Sans threatened, staring down at her with a predatory look of hatred in his sockets. Maybe if he was just given a reason for what she did, perhaps his past would be less haunting. It wasn't a matter of, if she told him then all would be forgiven. Because that wasn't the case at all. He just knew that maybe if he knew what the hell happened, he wouldn't have to care about it so much anymore. 

"You want me to answer that now?!" She saw the look Sans gave her, that almost looked like he was threatening to drop her. "Look... I'll answer your damn questions when we get the hell out of this, okay?! Now pull me up!" She said, seeing Sans debate it for a second, until he finally did decide to pull her back up onto the roof, stopping her from falling. "Fucking go!" Blair said, giving Sans a slight shove as the two of them already started running towards the other roof to take a jump. The both of them leaped, thankfully being caught by the others and that's when they were getting pulled up, just as the door on the other building burst open. 

"Oh shit." Asriel spoke up, staring towards the roof they had just abandoned, and that got Sans and Blair looking towards where they had just ran from, and that's when everyone's eyes widened in shock. How the fuck...?

"Okay, this stops, right now!" Undyne screeched, as none other than Gaster, Grillby, Toriel and even Mettaton were being restrained at her side by other FBI members. They looked like they were wearing handcuffs, which must have been made to stop monsters from using magic. Otherwise if a monster could just use magic when being arrested, then they would easily get away every time without question. "I am done playing games. I gave you an offer, and that was to come quietly, but if you can't do that, then I will use any means necessary to get you to turn yourself in." 

"HOW DID YOU--?" Papyrus said in complete shock.

"There were more of them than we expected. We were overpowered." Gaster simply stated with a calm sigh. He did seem disappointed, but he also was weirdly calm for someone who was currently being restrained. Maybe because he just knew that even if he wanted to do something about his current situation, he was unable to do so. 

"It's all over, Sans. Hand yourself in, because I do not want to do this." Undyne threatened, and that's when she pulled out a gun and held it right to Gaster's skull. Gaster just looked towards Undyne out of the corner of his eye with that straight faced expression of his and then his pupils trailed towards the others. They all looked horrified. Toriel, Mettaton and especially Grillby were also terrified. The skeleton weakly smiled towards his scared son. Sans was one who normally really didn't like to show fear to his enemies, but this was one of those moments where he couldn't stop himself from showing it. 

"Sans, it's okay. You should go." Gaster said. "I've already lived my life, at least as much as I can live it anyway." He said, glancing towards Grillby. "Even though I know there is probably a lot more I would like to get up to, it's not worth putting yourself in prison for. So, go." Gaster told him, looking back at his oldest son who slowly shook his head. Was his father really just asking him and everyone else to abandon him with the possibility of Undyne shooting him in the head? True, there was the very good chance she was bluffing. After all, she was an FBI agent, not a murderer, right? She wouldn't just do it, would she? 

"...fine. ya win, undyne." Sans exhaled, already seeming ready to put his hands on his skull. "i'm willin' to come quietly, as long as no one else gets hurt..." He muttered, seeing Undyne look proud of herself as she lowered the gun, feeling like the skeleton had made a wise decision. Everyone else looked at him as if he was insane, but he just knew that he couldn't take the risk. He couldn't. 

"We'll send a helicopter over to pick you up." Undyne simply stated. "This is probably the wisest decision you've made so far." 

"Well, if he is going, we are too!" Frisk spoke up, seeing Sans turn to her with an expression of horror and she looked back at him, sadly smiling. "I'm not letting you go to prison alone. We either run away together, or we go to prison together, and I'm not going anywhere alone without you." She said, going to go and reach for his hand. It was then that once again, deciding to stand up for him, everyone else in the group were saying that they were also willing to give themselves in. Even Blair. What...? 

"but, i..." Sans started, but then he wasn't even sure of what to say. He just sighed and tightly clung onto Frisk's hand, knowing that somehow... They had to get through this, right? They had to. Because he wasn't sure if he could handle their futures getting ripped from them like this. There had to be some way to get themselves out of this.

But for now, they would just have to play by Undyne's rules.