It's been (probably) hundreds of years since the end of GoW 3 before GoW (2018). At the end of GoW 3 he even realizes that his actions fucked everything up, and tries to kill himself. I think it's fairly believable that in hundreds of years of wandering and reflecting, and getting older, that he grew as a person beyond just being angry and wanting revenge (although that is still there and he fights against it every day). Not to mention it's heavily implied that meeting Faye had a lot to do with it.

Sure, they changed his character. Personally I think they changed it for the better.