It's not a mental health problem. It's a propaganda problem. These people aren't clinically insane. They are just enveloped in hatred. In particular dehumanization propaganda brainwashed them into seeing others as not even human.

If you go to places with decades long conflicts, you'll see this mentality on display everywhere. So it's not a mental health issue. All these people aren't insane. They are just really convinced that some other people are the enemy and that those other people are not even worthy of existing at all and that their life is worthless.