There was a time- not very long ago for one, and a lifetime for the other- when Steve Rogers would have gladly died for Bucky Barnes, and vice versa.
I am with you 'till the end of the line.
One of them remembers this, as he throws away his shield- what use is it to him anymore? Captain America's job is done; he's saved the world, again, saved everyone- with the notable exception of the one he'd failed to save so long ago. The handle gives way with a snap; a scream, and then he falls into the icy abyss below. This time, Steve Rogers won't fail Bucky Barnes. He can't.
With you 'till the end of the line.
The other is trying not to understand, despite the way the blonde man's face seems to pulse with familiarity, a forbidden thought scratching for attention in some closed off corner of his mind. He tries to silence it- focus only on the mission, the mission is your only purpose, finish it- as The Winter Soldier's fists smash in beat into Captain America's face, skin and bone breaking beneath his knuckles. He doesn't know this man that he is killing. He doesn't.
'Till the end of the line.
One cannot leave, not now; he cannot even raise a fist, because he is saving him. It is more important than anything else, that Steve Rogers saves Bucky Barnes. He lets the other's metal fist smash into his face, and rage fuelling every blow, the Soldier beats the Captain, his mission, until he is broken and limp upon the ground.
Through bloody lips Steve whispers, "I am with you 'till…"
The end of the line.
The ground gives way with a metal scream; a terrifying silence and then-
One of them falls, bloody, beaten and bruised, and is swallowed up by water and debris.
End of the line.
…
…
…
The other jumps, and saves him.
...
Have some Cap 2 feels from me. Also expect a spike in activity because I am finishing a bunch of fics that have been sitting around for ages in my folder.