Imagine this would have been Bucky's last moment...
When Steve gets up and leaves Tony behind, he wants to help up his best friend but then he hears Bucky's quiet voice, gargling and blood dripping from the corner of his mouth. "S-ste... vie" before Steve sees the life fading from his eyes, the bright blue turning into a dull grey, staring off into nothing.
"Buck...? Bucky - no... No no no, y-... You promised to be with me till the end of the line! Open your eyes, c'mon! B-b-bucky, p-ple... ple-ease" Steve sobs, feels tears rushing down his face and wraps his arms around the still warm but limp body, not caring that he smeared Bucky's blood over his own face as he hugged his dead friend.
A part of him noticed Tony struggling to get up to his feet and limping over to him, but Steve didn't care. He lost Bucky... the last connection to his past... With shaky fingers he closed Bucky's eyes to not see the accusal in his eyes - why didn't you save me? Yet again?
"Cap, I..." he heard Tony's voice but Steve cut him off, not letting go of the slowly getting colder body of the man he had been having even when he had nothing.
"Y-you got w-what you wanted, Tony" Steve said trembling, not angry, no, just defeated. He felt drained, hurt, oh he had never felt so hurt in his life before - despite the last time he thought Bucky died. But this time he knew it - he was laying dead in his arms. His heart was shattered, his sight blurry from his tears and his body shaking with each sob.
"C'mon, f-finish it... P-please, Tony, finish it..." Steve begged and cried into his best friend's chest, hearing no heartbeat. He couldn't bear losing Bucky another time; he'd break - he wanted to be with him. Forever. And he only saw one way to be reunited with his best friend...
"Please..."

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