I find the concept of an eye for an eye to be very inaccurate, and was partly how the movie romanticizing organized crime.

In real life, plenty of dons ordered killings that had nothing to do with avenging murders. And I'm sure each of the 5 families, at one time or another, had mobsters accepting murder contracts for money....and these contracts didn't have to include avenging a murder.

You shit-kicking, stinky, horse-manure-smelling motherfucker you! If you ever get out of line over there again, I'll smash your fucking head so hard you won't be able to get that cowboy hat on. You hear me? Fucking hick. -Nicky (Casino)