She avoided the destruction to happen on the ground which would have caused more deaths. It's called a character arc. She felt guilty but then she developes that feeling to embrace the responsibility of being a super hero. Cap supported her, Tony tried to enclose her (also she blames Tony for what happened to her parents).
Did I feel that a Cap movie felt like a Cap movie? Yes I did.
The damage is already done and that isn't gonna get changed by a letter. I also think it actually improves the climax since he acknowledges their different points of view while also asking Tony to keep fighting for what he believes is the right thing.
If you expected two characters to fight to death because of a missunderstanding without being fully comited to their cause and ignoring all the past character development and history they had together, this simply wasn't your movie.
Also, in the Airport only 3 people made jokes. Spidey and Ant-man because they are the funny ones and they werent that invested in the confilct. And Tony, which we've learnt he uses humour as a way to hide his fear in battle. (You could include the Hawkeye and Black Widow exchange, but as I said, if you were expecting these characters to fight to death and to ignore they had history, then this wasn't your movie).