Hi.I have The Mafia Encyclopedia. If you notice there are slight variations even within that book concerning the reasons. What's common though is that he was suspected of being a rat and of stealing money. Either one is a death sentence.

Stefano was the worst but Buccieri, who was closer to Giancana than Stefano and was just about as sadistic and murderous as Stefano, was also involved in the torture-murder along with Jackie Cerone. It was evidently Cerone and/or Buccieri who came up with the idea of the cattle prod. Supposedly Buccieri took pictures of Jackson's mutilated corpse and passed them around.

The source for Buccieri's claims is evidently an FBI wiretap.
Jackson was a collector for one of the Outfit's loansharking operations. Loanshark loans (or occasionally a specific type of loanshark loans) are occasionally referred to as "knockdown loans", hence "knockdown artist".

I don't know if Jackson was an informer or not but to label him one is a form of dehumanization which allows people to justify such incredible savagery-not that people like Buccieri or Stefano need any justification.

"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.