Joe Ianuzzi said Tommy Agro told him he killed DeSimone , a possibility as he had already murdered DeSimone's older brother and was known to joke about killing the third DeSimone brother and completing the DeSimone trifecta.
When Henry Hill turned informant he implicated John Gotti as the killer and reaffirmed it twice more after that. Allegedly, Paul Vario was having an affair with Karen Hill while Henry was doing time and Tommy DeSimone tried first to seduce and then rape Karen, after learning this, Vario then informed John Gotti about the murders of a Gambino soldier and of Billy Bentvena who were a close friends of Gotti's. I believe that Agro also stated that while Gotti was present, it was Agro who killed DeSimone very slowly and painfully. I couldn't see John being there and not getting a few shots in for his slain buddies but maybe he didn't want to get dirty.