Seems they were arguing a lot and she admitted she was a perfectionist. It was a small van and he didn’t look anywhere near a perfectionist type.
He probably got drunk and she pissed him off. He went ballistic and killed her.
Apparently according to Gabby’s friend Brian heard voices and had trouble sleeping. So maybe one of those voices started telling him she was the devil.
According to some reports I'd read, her friends said he was very much a control freak with her. There was also a report from a witness to his physically hitting her, so there's that.