Why did Jack kill her was it over her sleeping with Zeke?
I've seen a pattern of Jack's homicidal tendencies.
If you've noticed, whoever knows about Jack's behavior, his secrets, or has been involved with Naomi Singer have all been killed. So this could give us clues on why he's killed everyone who was involved with her, or why he killed Naomi Singer. (But this is all from the Unrated version, the killer was later revealed to be Zeke)
At the end of the movie: Carly found all the tapes of Zeke having sex with Naomi Singer. This could give you the answer that Zeke has indeed slept with Naomi.
I think Jack knew about this affair. It's been known that Jack is a very jealous and pitiful man. And that every woman he was involved with were murder all because they all slept with Zeke. Both Vida Warren and Naomi Singer. Guss Hail was killed for spreading "gossip" to Carly about Naomi's death. But the fact that he was a very "close friend" with Naomi was the reason why Jack killed him in the shower.
So there you have it. I didn't mean to spoil it, but there's the answer.