Originally Posted by
greatmetropolitan
Stark and Walker have similar arcs in their stories leading them to trying to/killing a guy - they're both basically humiliated from the very beginning, and you see it take its toll. Stark starts Civil War hearing about the death of a kid due to the events in Sokovia, tries to do what he thinks is the right thing, alienates half his friends, forcing the retirement of Captain America. He then gets a dressing down from Captain America for effectively interning Wanda. And this is of course all wrapped up in major daddy issues. Oh and Cap knew his dad, to make it worse. He then shows up at the airport expecting basically a pretty quick victory and gets humbled again, his miscalculation causing the serious injury of his best friend. THEN he finds out he was wrong about Bucky. Then he finds out Bucky killed his parents. Its actually perfectly built up for Stark to lose it.
Walker, by comparison, feels like his road to murder is built purely on ego. He's supposed to be Captain America. Cap's two best friends remind him he is not. Then he gets schooled by two normal humans. Takes the serum and still can't save his best friend. For Walker its all about him, his ego, his respect.
The difference between the two men is, for me, that if Stark had time to process and cool off, he wouldn't have tried to kill Bucky, but Walker seems to have doubled down on his belief that yeah, it was totally right to take that dude's head off. One man is trying to be good, one man thinks he is already and shouldn't be questioned on it.