There are no doubt that a lot of the killers in the mafia are psycopaths. But we must also remember that these killers are used by the mob just like state will use an army and the police to protect its interests.

The brutality comes from the fact that the mafia doesnt own any prisons or write any laws.

Take banking as an example. In the real world, if you borrow from a bank and dont pay them the moneyback, the bank can use the police to enforce some kind of repayment. But if you borrow money from a mafioso and dont pay him the money pack, the mafioso need to hire some guy to come after. And he will be brutal because they want to send a signal that you do pay the mafia what you owe them.

I think these violent soldiers of the mafia are just that, soldier. At any time, them mafia might dispose these psychopaths if they become a nuisance or doesnt obey orders.