I just finished "Unfinished Business". Pistone definitely has a healthy ego and is extremely supportive of law enforcement but that's not really surprising. As far as the supposed contract, I was initially dismissive as well but given that the Colombos were actively trying to kill federal prosecutors and FBI agents after Pistone had completed his undercover work, perhaps an attempt to get him wasn't so far fetched??

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.