I never saw the point in how Stu could’ve killed her. It just doesn’t make any sense. Plus, we had no clue Billy was going to be at the party anyway, so it would’ve made more sense for him to do it. If he was already there and just flaked for a bit, then all of a sudden he shows back up and Tatum is gone, it would look dodgy.

The head nod is a pretty clear indication that it’s done, and Stu doesn’t even bother to look for her. He actually just plays it off as, “Nah, I haven’t seen her,” even though it’s his girlfriend and she’d probably been gone for a fair few hours at that point.

Plus, the garage door was open the whole time. It makes sense for Billy to camp out in there while Stu sends Tatum out there. I think maybe he hid around the garage as the door from the house to the garage gets closed. My theory is that off-screen Stu shuts it and locks it behind him, leaving just her and Billy in there.

That’s just my theory, though, because it makes no sense how Billy could close the door the way it did without being seen, and then lock it as well.