No one. He has never personally killed anyone.
As for the murders he was involved in, there is the killing of his best friend Larry "Champagne" Carrozza. He had an affair with Franzese's sister, who was married.
Michael Franzese didn't mind but i guess it was too much for the Colombo family who could not tolerate that one of their associate was having an affair with a woman who was respectively the daughter and the sister of two made guys. Franzese was not part of the hit team but he knew that Carrozza was going to be killed.
Franzese was en earner but never a killer. Nothing wrong with that as far as the Mob are concerned. The Colombo family had enough killers to do the job.