It's a decent ending, fits the story... But i'm still kinda disappointed Kitami really died in the end.
He kinda had to. He was the shadow behind Himari's light, and used violence and vengeance to fuel his revenge on all of their classmates in Himari's name. But he turned into the thing he hated as a result; if he'd lived, he would've been locked up like their sensei was, which wouldn't grant him true release from both his past and who he'd become.
Him dying was both absolving him of his own sins, while giving him a clean way out. That he did it whilst protecting Himari is even better, because it closed the book on his original wish: to protect Himari. Through his death he cleanses himself, and becomes a symbol through which Himari can fully take ownership of her own life while at the same time tethering the rest of the class to her future. Plus, Himari could remain the pure part of the duo, who could shine and be happy and become the best version of any of them, which is all Kitami really wanted (hence his parting words to her that she recounts in the interview here).
Thus, Kitami surviving means that he doesn't get his image washed clean by his act of sacrifice to save Himari, and all of his revenge comes to light which would hurt Himari by association, as well.