There's some context to it in the story. They set Batman down that path in Arkham City's DLC 'Harley Quinn's Revenge' (which I believe also was not written by Paul Dini). I think you can interpret Batman as depressed, which does make sense given that he's faced with the prospect of losing his mind. In the background text in the game I do think they mention him becoming more withdrawn.
It is a character assassination besides that though, anyway. The Arkham Knight is completely justified in his hatred of Batman, given how quickly Bruce Wayne moved on.
It is a character assassination besides that though, anyway. The Arkham Knight is completely justified in his hatred of Batman, given how quickly Bruce Wayne moved on.