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?
Interesting question. Now and then, a killer is both mass murderer and serial killer. I did a post about Mark Barton, the day trader who massacred several people in Atlanta. But he might also be considered a serial killer, since he murdered his first wife, and then, the day of the massacre, his second wife. See details here: Atlanta massacre
Of course, the FBI has detailed definitions of both.
Mass murderers are event-creators. Serial killers are creatures of habit.
I just added a new post a to my Renzi Rant blog about mass murders. There are several interesting links within the piece to online articles I used as source material. The title: is below, with the link.