We don't know.
But like
@MassGuy has said, he doesn't present as a mass murderer, fits better eith a serial killer which he may have gone on to become if he hadn't been arrested after his first crime (quadruple murders but singular crime, in a sense).
Multiple victims in a singular structure isn't unique, however. He may have been a student of Bundy.
It's the backdrop that suggests a singular target. Hard to anticipate KG would be there. Hard to anticipate XK was going to be awake. I think he reconnaissance was poor. And he went from active to reactive, gunning down everyone in his way.
I think he encountered MM at the Mad Greek. And encounter that probably barely registered for her but fired a fixation for him. Followed her home, more than once.
He knew exactly where that house was. 23 times, leading up to 11/13.
JMO