You've never properly fought Steve; he'd always pulled his punches, slowed his reflexes, and given you a fighting chance. The fights had always been games with undertones of humour and flirting. You had always enjoyed sparring him.
This is, well, hell on earth.
Even with force-fields to take the brunt of the impact, his hits land with brutal force and accuracy. Steve has abandoned his shield and is attacking you with just his bare hands. Even with him slowly beating the life out of you, you can't find it in yourself to properly fight back. For one, it's taking all your focus to keep his assault at bay. Secondly, it doesn't seem like there's a real point because he's Captain America, possibly the best soldier that the last two centuries have seen; you know that if you can't break the trance that this is a fight he will win. And finally, possibly most importantly, you love him too much.
"Steve, stop! You can fight this!" You try to break his reverie between blows. "You are the strongest person I know, you can do this! Please!"
You continue to beg but the words all fall on merciless ears.
—
You're starting to slip up; your shields are becoming weaker with every hit as your energy and strength drains. You've already fought your way into the facility through a sea of enemies, and now you've barely got anything left. A hook slips through your guard and lands in your ribs. Sharp cracks followed by blinding, blooming pain that flashes up your left side saps whatever power you had left. Your force-fields drop and the onslaught truly begins. With what you have left, you place a small shield over your head in hopes of stopping any killing blows. Unfortunately, this doesn't do much to stop him. Steve aims for your ribs, battering you until the world is only made of razor-sharp pain and dancing lights across your vision. You can feel the force-field slipping, so you do the only thing that there's left to do.
"I love you, Steve Rogers. More than you can know."
The shield slips and you see him wind up for the punch. Despite everything, you work to memorise the details of his face. Despite everything, you still love him so much that it aches. Despite everything, you wish you could stay because God damn you want to kiss him one more time. Just before his fist lands, you hear an electronic beep go off like the end of a timer. The world goes cold and black before you can consider what it means.
Steve's world is blurry and muffled like everything is made of frosted glass. He can't remember where he is or how he got here and the only thing in is his whole headspace is Y/N; how warm she felt pressed against him, how she cradled his face like something fragile, how the hunger on her tongue drove him insane, and how much he wants to touch her again. Suddenly, his mind starts to clear. He's in the facility, a shrill beep is filling the air, and something warm and thick is coating his knuckles. As soon as the world comes back into focus, he sees that Y/N is in a bloodied pile before him. Something in him instantly cracks.
She's dead.
He wants to kill himself for even letting the thought cross his mind but his eyes can't escape the truth. He leans forward to pick Y/N up, cradle her, check for a pulse, but before he can Steve sees his hands. His knuckles are covered in blood, though not his own. His palms are mostly clean so it can't have come from helping her. He had never got close enough to the scientist to hit with anything but his shield. But how the-
The device that wasn't a bomb. The chemical burn that wasn't a burn.
No. It couldn-
No.
No no no no no no no no-
He picks Y/N up as gently as he can manage. She's breathing (just) but there's a choked, fluid sound to it that suggests her lungs are punctured. She's unconscious and her pulse is soft enough to be missed. Her skin is becoming colder to the touch with every heartbeat that drains her life onto the floor. Y/N is dying, her foot further across the threshold towards gone than still here. And worst of all…
I did this to her. I killed her with my own hands.
The true realisation and atrocity of the moment cascades over him like a breaking swell, repeatedly and with devastating consequences.
"Y/N, please wake up. I'm so sorry, please." Steve's words are incessant, pleading, broken.
He says it over and over again like a skipping record until sobs swallow his words. He cradles Y/N tightly against his chest and buries his face into her shoulder. A howl full of his anguish, his guilt, his loss rips out of this throat. Steve tenderly lowers her until she rests on his knees, her face still and calm beneath the blossoming black bruise. One hand to steady her, he places a palm against her cold cheek, covering the mark even if just for a minute. Tears still steadily course down his skin, leaving trails through the dirt and blood.
"Y/N, I'm begging you, don't go where I can't follow. Please, I love you. Please, just- just stay."
"Please Y/N, I don't know what I'd do without you."
Y/N stays quiet, unstirring, and Steve's fingers trail down to her neck. It's at this moment that something which had survived the brutality of war, the loss of everyone he loved, and the cruelty of time quietly dies within him.
There's no more pulse.