I am thinking kidnap gone wrong, I have a feeling that he has tried to scare her and its gone horribly wrong. At this point we don't know much about his character, but he is a family man with a small child. Obviously I don't know him, but he just doesn't seem like a cold blooded killer. The whole scenario doesn't seem like its been thought out well.
We know he threatened her, but no one has confirmed these were "death threats" maybe hes threatened to harm her over the issue, he's turned up at her house to scare her, hes tied her up and lead her away somewhere and she has suffocated to death. In a panic he has then called his father and friend and they have come and helped dispose of the body in the quarry. I wonder if he has confessed to someone? Maybe Katie? and thats how they pinned him down so fast and also knew it was kidnap?
This is in no way defending the actions here BTW, But just some food for thought. Maybe this guy began with the threats and bitten off alot more than he could chew.