I am leaning toward him using the gun as a kidnapping tool. We know he points it and then says GDTH. I questioned earlier why we're not hearing about shots being fired from witnesses.
I've come to the conclusion that there were none. As I said, I believe the gun was to threaten them to get them DTH so he could... do stuff to them.
We know there was a lot of blood at the scene. Since there's been talk of a knife in both RLs PC, and RMA's as well... I think he used the knife to kill them. It's quiet (except for screaming) and likely why his clothing was bloody (frontal spray).
What I don't like about where my mind is going with this is if there was no sign of a struggle or a fight, and their clothing was removed, is that he killed them first, and then did whatever perverse thing(s) he did. Because I can't imagine them not struggling and putting up a fight at the point where he's trying to get their clothes off. Sure, they could have been frozen with fear but I don't think so. I was almost raped at 15. I was passed out on a bed in a basement. I could only muster an "Off... off...". He didn't get off so even in my very drunken state, I thought about what was close at hand. The nightstand! I grabbed the lamp and hit him over the head with it. Yeah, I've posted before in other threads where I tend to get angry when ticked off. Abby and Libby weren't passed out drunk. I'd have thought they'd have fought or run. I know I would have. With all that said... this just came to me. Perhaps the gun is what kept them from running off.
It's all so very sad.
Also, I can't imagine the girl who wasn't not having her clothing removed at the moment to not hightail it out of there. I know, some think she'd have stuck around for her friend but I'm not so sure. I think their deaths were quick once he got them down the hill (and why no signs of a struggle), then removed their clothing and did whatever he did before getting the heck out of dodge before someone saw him. What a disgusting human being.