Maybe I'm just an unemotional sot but.....I'd seriously realize Barnes was more-or-less programmed to be Winter Solider before I'd go off the deep end like that but overall I did like him in this,it's what Tony was in the comics somewhat a good guy basically but has his problems and in this case is trying to keep the ship level and wound up capsizing it.
I think the Russos went to great length to make it plausible by putting Tony in an emotionally fragile place where he would be less than rational.
Right from the start Tony and Pepper are not together. In the MIT lecture you can see that he's actually still shaken by it. Let's remember her role from IM was as an ally for his conscience, albeit a conflicted one. Without Pepper, Tony only has Steve and the Avengers.
So the Avengers is more than just a team for Tony, it's a surrogate family which is even more vital without Pepper in his life. With the rift over the Sokovia Accords Tony's appeal to Steve is about keeping the Avengers together so he doesn't lose his family. In the airport scene when Tony says, "I'm ... I'm trying to keep you from tearing the Avengers apart " he's on the verge of tears because he's really emotionally isolated (with the exception of Rhodey). When Rhodey goes down Tony is suddenly without anybody. The Vision is still emotionally primitive while Natasha essentially abandons Tony she tells him he should watch his back. Tony is alone.
Now let's talk about Tony's relationship with Steve. It's almost like a sibling rivalry since their common link is his father. Tony never really got closure with his dad. In Iron Man, when Tony says, "I never got to say 'good bye' to my father" and repeatedly speculates about what his father would have done you really get the sense of him desperate to fit into his father's shoes - a boy still in want of approval. At the same time Tony grew up hearing his father constantly lauding the memory of Steve Rogers and, in hearing his dad talk about Steve, Tony even says, "God, I hated you". At least in Tony's mind Steve was the "golden child" and he was obviously the "problem child."
Even with Steve representing a moral high ground Tony goes to great length, in different ways to convince Steve to sign the Accords. We already saw that he fears the dissolution of the Avengers. In the scene where he brings out the fountain pens he even reveals to Steve that he hopes the Accords will help bring Pepper and him together again. Despite all this Steve just stands his ground unsympathetically. While Tony is being uncharacteristically humble, Steve is kinda being a dick.
Even as Tony pleads to preserve the Avengers, Steve seems more committed to protecting Bucky. I sense some jealously in Steve's dedication to Bucky over Tony. ("He's my friend." "So was I.")
Seeing the video of the murder of his parents - his father with whom he was never able to express affection and his mother who coddled him (let's face it, Tony is a "mamma's boy") by the singular man who is being protected by Steve causes him to lose control. In part he's also taking out Bucky in retaliation against Steve who seems not to regard Tony's situation at all, who was worshiped by his dad and who is protecting the murderer of both his parents.
Throughout the movie Tony is struggling to remain cordial and find a way to resolve this with their friendship intact. The video was the exact thing that could break him.
