Eddie Kane, in Publish or Perish wants to kill lots and lots of people. Riley Greenleaf is so aware of this that he specifically warns him that he is not to kill anybody but Alan.

If you mean people who have a motive to kill someone in particular, my first choice has already been mentioned. Margaret would certainly hate Leslie for killing her father.

A Deadly State of Mind is full of possibilities. Karl Donner might want to kill Nadia for sleeping around on him. Nadia might eventually want to kill Dr. Collier for having taken advantage of her in a therapy relationship.

Regarding Nadia, I think it would take some sort of psychotic break for her to want to kill anybody. Maybe if she believed that what Dr. Collier did to her was a terrible betrayal and he started to emotionally push her away.

How about Arthur Kennicut from DEATH LENDS A HAND? Brimmer lied to him about his wife's affair and then killed his wife whom he professed to love very much.

Luke Basket from SWAN SONG. He punched Tommy Brown while accusing him of murdering Edna, in front of a homicide detective, no less. I think he would have eventually resorted to murder himself if Columbo hadn't caught Tommy.

Curro Rangel from A MATTER OF HONOR. He seems hot-tempered and if knew earlier that Montoya had killed his father, he probably would have gone after Montoya.

As far as you can tell, Karl IS trying to kill Nadia. Or at the very least, what he's doing could lead to it. That's why that one always leaves a funny taste in my mouth - no matter how much of an opportunist Dr. Collier is, he's still preventing Nadia and himself from getting killed, at least as far as you know.
Of course, that's why (except for Death Lends A Hand) the show always follows a killing like that one with a premeditated one.

New to this forum so not sure if this has been mentioned but...

-Swan Song: Edna's brother Luke punched out Johnny Cash when he was playing guitar and in crutches just after a huge accident! Right in front of a cop! He definitely could have done worse.