You guys should check out the movie "At the Sinatra Club." It is also a book. The writer is Salvatore Polisi, who at the time claims to have been a Colombo Family associate. He also testified against Gotti in '85.

He says Gotti killed DeSimone for killing Foxy Jerothe, a made guy named "Melons", and he generally didn't like him. Plus, if DeSimone killed William "Billy Bats" DeVino, there is more motive. Polisi and Henry Hill knew each other, before and after they were both in witness protection. Polisi claims the all those guys knew each other and hung out together in the late '60s and early '70s as young men.

It is also possible that Thomas Agro and John Gotti, both made Gambino members, were both present when Tommy DeSimone was killed. His body has never been found. Lots of crews had reasons for wanting him dead, Luccheses, Gambinos, ie. So, to me, one, some, or a bunch of the motive theories could be true.