“What truly hits hard about Civil War’s ending is that it fully commits to the futility and deflating pointlessness of this final brawl without any qualifiers or moral escape hatches. Zemo hasn’t “tricked” anybody. The tape isn’t doctored. Everyone is on the same page as everyone else. Stark is 100% aware that Bucky was effectively being remote-controlled during the killing, and he (and the audience) hear Cap explain that, while he did learn that HYDRA had assassinated Howard and Maria Stark during the events of The Winter Soldier, he did not know that Bucky himself had done the deed. The cards are all out on the table very clearly indicating that, by both moral and narrative logic, these guys should not fight.
And they fight anyway. Because they’re angry, and because Zemo was only able to break The Avengers in half because they were already starting to crack. Because all that pent-up pain has to go somewhere, and because humans and their emotion-driven responses can’t be held completely in check by even the most well-meaning attempts at regulation. Even as Cap tries one more time to make Iron Man listen to reason about not murdering Bucky Barnes, when Stark responds with “I don’t care – he killed my mom,” we understand where he’s coming from, and why all the reason and logic in the world won’t stop him – any more than the Accords could’ve stopped Captain America from conducting his “vigilante” superhero work.”