None of them are important to any quest as far as I know, so killing them should have no consequences.
Cingor in Leyawiin; however, is part of the Fighter's Guild if you don't have the Unofficial Patches installed. So if you kill him and you're a member you will be expelled.
You'll definitely want to get the patches as they fix thousands of bugs and oversights like this one. Cingor has no business being part of the Fighter's Guild. He never even goes to the hall I think, nor do any Fighter's Guild quests involve him.
If you don't have the patches, I advise letting the town guard take care of them just to be safe.