A lot of people participated in those murders but Frank Calabrese was definitely one of the core group and pretty much the worst of the worst. That's even what Judge Zagel told him at sentencing. He essentially said the other men had some redeeming qualities but that Frank Calabrese was a monster. I think he referred to him as a "heinous creature" or something. Look at it this way, his own kid turned him in, just because he thought his father should not be let loose. Then his other kid essentially testified to the same thing. And of course Nick turned on him. The FBI agents working with Nick said they didn't think Nick would have ever turned, at all, if Frank hadn't been such a rotten person.

You could say that Frank really brought down a significant part of the Mob just by being such a rotten SOB.