Many known bosses had murdered when they were younger. Example: the Chin, John Gotti, etc.. but the only ones who killed WHILE they were boss were the psychos.

Yes, I believe if Casso had been made boss he would've continued killing people personally, but that's because he was a psycho and killed people for fun. I think Anastasia might have killed someone as boss, but once again, total psychopath.

Just like you get a lot of pedophiles working in schools (sorry, disgusting I know), you get a lot of sociopaths choosing to be in the mob so they can murder and do other horrible things without repercussion.