I think the current kerfuffle in the marvel fandom owes a lot to the fact that most of the fandom hasn’t read the comics. That allowed for the mental creation of a Bucky Barnes that is unrecognizable if one HAS read them–a deeply damaged, non-functional man who doesn’t want to hurt anyone, and is more likely to cringe or weep than fight, so it’s really shocking to those who believe in that model to see Bucky shoot someone at all, let alone at point blank range.
To those who’ve read the comics, that’s not even remotely surprising. They’ve seen it before, especially when it’s people who’re trying to harm Steve. You harm someone he cares about, you die, and the only people who can stop him are those people he cares about.
The Winter Soldier isn’t a separate personality that can be expunged from Bucky. The Winter Soldier is Bucky and Bucky is the Winter Soldier. That doesn’t change. What changes are his goals. His methods, not so much, just degree to which he uses them and with situational checks and balances in mind.
If it WAS someone other than Tony that he was initially aiming at, then Tony is between him and his target, and the only way he knows Tony is as a guy who’s siding with people who are trying to destroy him and Steve. He’s not his friend, he’s an obstacle or an enemy.
Bucky isn’t fandom. He cares nothing about Tony except stopping him. From his POV, why wouldn’t he shoot him? Something bad has clearly happened and he’s focused on removing a threat. He will do anything to protect Steve, and to a just slightly lesser extent, to survive. The only thing surprising here is that fandom is surprised. You saw the Winter Soldier in action in CA:TWS.
They were really clear in interviews, and the comics are equally clear–prewar Bucky is gone. He retains elements of his personality, but he’s not the same guy. Not only is Winter Soldier Bucky not a cinnamon roll, he never was. He’s a complex person who makes rash decisions and will risk certain death for the people he wants to protect–like jumping between Steve and a tesseract weapon in CA:TFA.
Thing is, the guy who he is now, he’s awesome, too. He’s an guy who makes mistakes because his heart overrides his head, kills more freely than Steve would like but is very, very good at it, is intensely loyal, and has many lovable qualities. In other words, he’s a guy with both flaws and positive traits, which is WAY more interesting than a perfect cinnamon roll. Fandom just doesn’t know him, yet. Well, except the folks who’ve read the comics.