One of the classic Bible murders occurred when David got his henchmen to arrange the death of Uriah. That left him free to marry Uriah’s widow, Bathsheba. James M. Cain used a similar plot (sans henchmen) in The Postman Always Rings Twice.
Dashiell Hammett’s The Maltese Falcon is a classic tale of greed. All the killing is done in an attempt to acquire the supposedly ancient black bird.
Another popular fictional murder motive is revenge or retribution. This has spawned the good guy killer fad seen most notably in Lee Child’s Jack Reacher novels. The hero doesn’t consider them murders but retribution for injustices to himself or other friendly characters.
Interestingly enough, this type of rationalization is similar to that of the schoolyard killers. At Columbine and Virginia Tech, the students rationalized that they were punishing other kids who had bullied them, ostracized them, made fun of them, or generally made them feel unwanted. In the fictional world, authors make sure their targets are painted black enough that there’s no doubt “they deserve it.”
Actually, rationalization is the balm that most murderers use to justify what they’re doing in their own minds, even when they know it is against the will of the law and society. They become determined to do it anyway.
I say most murderers, because there are always the psychopaths—serial killers. These guys (and a few gals) are so egocentric and socially disconnected that they know what they’re doing is right. Nobody else matters, so what’s to rationalize? Psychologists say there are plenty of them around. Fortunately, only a few drift into the murderous category. Except in fiction.
So why do they murder? If you’re writing a novel, they can do it for any good (or bad) reason you can dream up. Just try to keep it believable.