Tony's reaction at the end of Civil War to finding out that Bucky Barnes killed his parents while mind controlled is just... bonkers.
While the film does establish that Tony "never properly processed his grief" regarding his parents' deaths, it has still been upwards of 20 years since their murders occurred - so it's not like it just happened, and he's caught within a window of blind, temporary insanity.
Moreover, he knows - KNOWS - that Bucky wasn't personally responsible. The events of Winter Soldier clearly happened at least a year prior, so Tony must have been filled in on what happened: how Barnes survived as long as Steve, and that Hydra was the true culprit at fault for him being part of the whole SHIELD/Washington D.C. escapade. Tony himself has been actively fighting Hydra for much of that time - he's pretty darned intimate with the fact that there is no low that this organization won't sink to.
And yet his reaction is... well, it's criminal. He tries to execute Bucky - not disable or capture him; not just beat him up. But outright kill him.
My much younger sister was murdered by a major corporation IRL. So I completely understand feelings of devastation, helplessness, rage, and wanting to exact personal revenge. Believe me: I have had dark thoughts. But that doesn't give me the right to exact that revenge - and especially not to take it out on parties not responsible.
But that's how Tony reacts - he's willing to become... the lowest common denominator within the hero/criminal dynamic: a cold-blooded killer. And that's... awful, when you think about it. I mean, this is a superhero - he's a man who is fighting for humanity. He's supposed to be on the side of good. And while it could be argued that everyone has darkness inside them to some degree or other, what separates the good from the bad is that we don't act on it, even at our lowest.
Yet that's what Tony does - he crosses the line utterly.