Being fully aware and not caring sounds pretty mental to me, a normal reaction would have been regretful after the fact once the adrenaline wears off, this isn't the case.
Not necessarily. How many people who commit genocide or murders in wars lead normal lives if not caught? It's not like the guilt of killing people leads them to live sad lives in depression. Most of them probably wouldn't qualify as mental health patients.
Sad fact is that some people, whether through nature or nurture, just don't care enough about the suffering of others.