To your first point: frequent high levels of stress alters brain chemistry and definitely changes behaviour, often in a negative way. Short term high level stress affects behaviour very significantly.

To your second point: the brain is very complex and barely understood by scientists. You are reducing brain damage to the most extreme examples you see in the news, while there are many other manifestations of brain damage. I have brain damage which affects my cognition, I have to regulate the impulses that my brain receives or it shuts down. The only behaviour change you’ll find in me is that I am less engaged in a direct conversation and sometimes struggle to find a word that I wanted to say.

Just because a change in behaviour can occur with brain damage, hopefully does not mean that you meet someone with brain damage and immediately conclude that they’re a bad person. Hopefully you look at their actions and base your judgment on that. That’s what I mean when I say that actions rather than brain damage defines if someone is a bad person.