First, Italy is one of the few countries that does try to stop immigration, so I don't think the mafia is even necessary.
Justice is based on an individual level. Rapists can, and should, be shot. But the majority aren't. Those should be deported, with maybe some actual refugees being granted safety in a country not of their choosing.
Group punishment for what a minority does is a bad idea, and an immoral one. So is killing innocents on purpose. These are both evil. And that's ignoring the countless other evils that the Mafia does, including being much more 'skilled' at sex slavery. The mafia are by no means a 'lesser evil' than mass immigration.