Try this scenario and see if it fits:
Killer's story is true... to a point.
He thinks intruder is in bathroom and shoots through door... one shot, hitting Reeva in the hip.
Reeva screams and he realizes he screwed up. She tells him he's crazy, and she's going to call police, etc. Maybe they fought earlier.
Killer breaks in door to try to save her. She keeps berating him while screaming and writhing in agony, threatening to call police, ruin him, etc.
Killer panics, snaps, whatever, and shoots her dead.
He calls Stander, then goes back and rips remaining panels from door, unlocks the door, and carries her downstairs.
Yes, he thought she was an intruder, and yes he intentionally killed her.
Possible based on evidence?