How do you define serial killer? Does it have to be someone who kills to satisfy some perverse, sick twist of his mind?
What about someone who killed for a gang initiation, in a robbery, to control a drug territory, because someone dissed him, because a woman cheated on him, to control a drug territory - who might have killed six times for various reasons? Is he a serial killer?
Their crimes are committed as a result of a compulsion that, in many but not all cases, has roots in the killer's (often dysfunctional) youth, as opposed to those who are motivated by financial gain (e.g., contract killers) or ideological/political motivations (e.g., terrorism, democide). Many times this compulsion is linked to the individual's sexual drive.