I recently discussed this topic with a few others, briefly, in another forum, on another site. (Sorry to have to admit that). Anyway, I found the subject very interesting, and wished we'd discussed it in more depth. It got started by a writer looking for ideas, and morphed into others admitting they typically stuck to two or three reasons for murder in their books, although, not purposely. Which had me looking at mine, seeing as how murder played a role in every book I've written thus far.
So far, I've used:
A psychopathic abuser
Murder for hire
A crime of passion
Serial killer (another psychopath)
How about you? Do you find yourself using the same reasons for murder over and over, or have you been versatile?