The Winter Soldier returned at daybreak. HYDRA's mission for him had been clear from the start — leave no one alive. It surprised Bucky it had taken him this long to come. Had he been watching him? Had he seen how he carried everyone inside, clearing the field for the impending fight? Had he allowed Bucky to do that? 
To hope Steve would still have a semblance of honor after what he did... No.

"I always said I was with you till the end of the line," said Bucky sadly when he finally stood face to face with his former best friend. "I guess we've really hit it this time."

The two men looked at each other; their faces set aglow by the surrounding embers. Hours seemed to pass with each beat of Bucky's quickening heart, and then... all hell broke loose. 

Steve aimed his rifle, and bullets rained down on Bucky. He kicked up the black-sprayed shield buried at his feet, the one Howard Stark made for Captain America and which Bucky had retrieved from the lab below. Each bullet ricocheted against the strong vibranium.
Upon seeing his action was futile, Steve threw the weapon away and ran up to Bucky at tremendous speed. The shield caught the impact of Steve's body when he crashed into him, but Bucky still lost his footing and smacked against a rusted jeep. Steve snatched the shield away, nearly tearing Bucky's arm off, making him cry out in pain. It thudded to the ground, just a few feet away from where Peggy had lain. 

Bucky deflected as many of Steve's punches and kicks as possible, but it was painfully clear he was a lot stronger and faster. If he had any hopes of ending this, he had to act right now.
Just as Steve drew his arm back to strike again, Bucky dove and somersaulted away. He dropped the activated device Howard had been working on mid-roll, praying he had finished it before his death. There wasn't enough time to get to a safer distance, so Bucky lunged for the shield and swung it in front of his body, rolling into a ball to protect himself as best he could. 

A big white flash illuminated the red-yellow sky above them. A scream filled Bucky's ears, and then there was nothing but defeating silence. Bucky held his breath, too afraid to move. Howard had been talking non-stop about his invention. He'd called it an... EMD or EMP or something. It packed enough of a punch to take out any devices running on electricity, and it could even kill someone if used in close proximity to the human body. Had Bucky not used the vibranium shield, it would've been the end of the line for him as well.

A sudden grunt made Bucky stiffen. He peered over the edge of the shield to see Steve on his knees with his hands clasped over his ears. He couldn't believe it. How had he survived that?
A pair of blue eyes widened at finding Bucky.

"Bucky?" Steve lowered his hands. "Wha-What happened? Where... Where am I? H-How did we get out of Austria?"

What the hell? 

Bucky slowly rose to his feet. He cautiously walked over to Steve. When he reached him, he put the tip of his fingers against Steve's temple. What he felt nearly devastated him. 
HYDRA had put a chip in the smack center of Steve's brain. Howard's device had fried it, thus freeing him from Red Skull's control. Now, Steve's fractured mind was tying everything that had happened over the last months together in mere seconds. Bucky saw Steve's feverish eyes as he tried to grasp it all. 

"Bucky, what... My God, what did I do? What did I do?"

Bucky kneeled down in front of him. He cupped Steve's face and forced him to look into his eyes.  

"Nothing." His voice broke. "You didn't do anything, you hear me? None of this is your fault, Steve. HYDRA did this. Not you. You're okay, you're — no, no, listen to me, please!"

But Steve thrashed about so wildly, Bucky had to let go. He watched in agony as Steve scrambled up, crying and pacing and just mumbling incoherently. He couldn't bear it. Steve's guilt would eat him up. He had to do something. 
And... there was only one thing.
Bucky got up and stood in front of Steve. He pulled him close, stilling his shaking body. Steve clutched at him, desperate for support, but Bucky drew back a little to look into his friend's eyes. His hands held his face. The brief brush of his lips against Steve's almost seemed like a dream. 

"I'll never stop loving you, Steve."

A single snap. A single tear. A single broken heart.

💔💔💔

HYDRA did not triumph that day. They never would again. Arnim Zola was among the first to die. Red Skull was among the last. A few tried to take command, yet all perished in the same way, their fates sealed by the one person this all started with. 
Sergeant Barnes' loss, remarkably, only strengthened him. Propelled by the most powerful emotion that could drive a person, he lives and fights on. Because there will always be another fight to fight. 

Love moves the heroes of our grand multiverse. Love inspires others to act and do the best they can. Love has made and will always make... 
An Avenger.

Thus ended this episode of What If...?
Did you catch the words hidden in the text? You know the ones - those words ;-)