I absolutely see your point, but I think Historynut might well be right. The video stops shortly after DTH, right? Isn't it possible one of the girls said, "Wait, why are you doing this? You're the guy from the drugstore, right? Why are you doing this to us?" MOO but it makes sense to me.
At that point,
murder becomes an obvious choice to prevent the girls from identifying him, whether or not he had planned it previously. The girls recognize him and he feels he can't leave them alive. He might not have intended to let them live in the first place; we just can't know what he had in mind.
And
@Salah11, I see your point, absolutely, but from what I've seen of youngish teeny-age girls (from my wife's teaching career) I think it's entirely likely that
(A.) the girls went in the CVS routinely--shopping is lifesblood to girls that age and the place was full of cosmetics and skin-care products and all that stuff; and
(B.) the girls might well have noticed RA, like this:
Girl1: "Hey, the little short guy with the stupid beard is at the counter today."
Girl2: "Yeah, I'm not going to buy the (whatever) today, that guy always looks at us in a weird way when he rings up our stuff." (Strictly MOO and speculative but IMO certainly possible.)