Unless he felt some kind of remorse afterward. Put her somewhere she is sure to be found so she can be taken care of. Or he was leaving her there to brag about the power he had, that he could do something like that and not be caught.
Reading about Jonathan David Drew they said he didn't have the build up like most killers. He was reported as violently attacking girls when he was as young as 8 or 9. Usually they start with petty things like peeping or exposing themselves before they work up to actually attacking a live person.
Or harming animals. That's a big red flag, iirc.