I really don’t understand the logic behind saying Tony’s the one who ~saw Steve’s dark side~, as though that final brutal beat-down by Steve wasn’t in fact him taking advantage of that last opening Bucky gave him in a completely desperate attempt to disable the Iron Man suit before Tony killed them all.
I’m not saying Tony would have killed Steve and Bucky, my point is: Steve thought Tony was going to kill them. And justifiably so.
Tony’s the one who flew into a murderous rage out of love for his mother. We saw him go off the deep end to avenge what’s his. Between Steve and Tony, one of them did see the other’s dark side, but it sure as hell wasn’t Tony seeing Steve’s. Tony wasn’t ~terrified~ of Steve, no more than he justifiably could be while bracing himself for an incoming attack at his face. He wasn’t overwhelmed by Steve and Bucky’s love. At the end of the day, the movie explicitly established that Tony is the one who’s most likely to literally kill a man for someone he loves.
Tony fought because he was enraged, while Steve and Bucky fought to stay alive.