The Winter Soldier is a cautionary tale for sidekicks everywhere. Sure, you’re having a good time battling Natzis in your underwear but how about after you get maimed and brainwashed and turned into an international assassin? NOW how much fun are you having? Wait, don’t answer that.
This is basically what happens to poor old Bucky Barnes on his last mission with Captain America. He and Cap are lost at sea and Bucky’s gravely wounded body is recovered by some Russian black ops program which is nice enough to cybernetically enhance him, brainwash him, and nurse him back to health. He then spends the Cold War era behind the scenes as a Soviet assassin.
After the millennium he finally breaks his programming and retires from the assassination game. He even gets a chance to be Captain America only sixty years after his last mission with Cap. John Wick made professional killing look really cool, though, and in recent years Bucky has taken up again as the Winter Soldier just to keep edgy.
References: [marvel.fandom.com: Winter Soldier]
[hero.fandom.com: Winter Soldier]
Metahuman Database 