Right now I'm leaning towards
1. Random killer, possibly meant to rob the place but got spooked/things escalated much further than expected
2. Romantic interest of someone who was staying at the house - maybe the killer was mad that the Tepes provided a way out for his ex?
3. Romantic interest of the Tepes in a swinger type situation - I don't think they had anything planned for the night of the murder (weeknight, kids home), but maybe the killer had been to the house before and knew the code?
I've seen the "swinger" theory bounce around these threads and I want to clarify that I don't mean that with any judgment. I'm single and in my early 30s and you'd probably be shocked by how many young (or old!) well-off/professional married couples are seeking a casual third. In my experience, most are seeking a woman. In that case, maybe the murderer isn't a lover, but a lover's spouse or boyfriend?
I'm still leaning towards 1 or 2, but 3 has been in the back of my mind.
MOO and total speculation. And again, no judgement on two consenting adults getting up to mutually agreed upon antics, if that was the case.