There are serial killer crime novels. There are crime novels where the antagonist kills more than one person. I wouldn't lump the latter into the former. What's the difference? Here's my line.
1) If the protagonist offs a pile of people, then he/she is a multiple murderer.
2) If the antagonist kills over and over, then he/she is a serial killer.
Exception to 2): The killing is not done to satisfy some psychological condition.
That's about as clear cut as I can get. What about you?