Brain trauma manifests differently in different people. It can change a person so much so that who they were before the trauma is gone.
I met an older family friend who changed. He went from calm, compassionate, never raised a hand or a finger to anyone for 50+ years, then a really bad car accident. Then a bunch of episodes. Constant headaches, confusion and sleep deprevation. Then after a few years the violence started. It's painful for everyone, especially him. He's frail and his outbursts aren't these planned attacks, its just an angry (maybe helpless is a better word) and confused person constantly misunderstanding the world around him and years of frustration building up. It's like he recognizes you and knows who you are but he isn't him. It's hard to explain beyond that.
I see the same qualities in fighters a lot before and after big fights. And I mean see literally. It's like it's in their eyes, that lack of presence in the moment... glazed over almost.