Even if she went with him willingly, even if she did consent to sexual relations, even if she had a seizure and died of natural causes, then why wouldn't he take her to the nearest ER? Perhaps because he knew they would find date rape drug in her system? I think he left her out in the elements to destroy all evidence. Being a hunter, he would surely know that would be the fastest way. JMHO
I don't know how far the DA has to go to prove that she was murdered. I don't think the rape drug is going to get anywhere as it dissipates quickly in ones system, from what I undersatand. Even if they find some of the drug in JM's car or apartment, there is no way to prove he gave it to Hannah, unless there is residue in, say a beer bottle, with her saliva on the mouth of it that they found. With what Hannah was wearing, difficult to hide the bottle, and the checker would have likely said something if she saw JM and Hannah holding beer bottles out there (but then, maybe not). So the rape could even be out the window, though the focus would be murder.
But really, he could have taken her for a ride there, made his moves, she jumped out and told him to scram, and he just left her there alive and angry. Or elsewhere in the area and she made her way there. That JM was with her, maybe gave her a drink, walked out of the area with her, possibly that she got into his car with him, all could be proved with a little DNA like hair left in the car, and the surveillance tapes and witness ID, though not likely the drink part. So maybe she did have sex with him. Maybe it was consensual. Or maybe she sobered up when he made a pass and ran off and where she was found was where she ended up. Maybe some other creep found here; clearly no shortage of them around though at 2-3AM out in that area where she was found, it is really dubious that one would be taking a stroll like JM was in the mall. So I think that if LE can just tie JM to that place where Hannah was found, the circumstantial evidence would be sufficient for a murder convict. But if LE cannot, I don't see a convict.
Also, I think JM just left her there, because he just is too lazy to do anything else. Just pitch the body after he 's done, as he likely did to MH. Probably figured it would be a while before it was found. Didn't even bother to cover it. When you think about it, incredibly negigent. Same as MH's. Didn't bother to bury the pieces or hide in underbrush or woods, just left it in a ditch so anyone going by there would see a skull, quite tell tale. That he got scot free from MH's murder probalby made him so careless with this one. To me, it's all impulse driven.
So a lot can be riding of Hannah Graahm's remains beyond simple ID. Show she was murdered, show that JM was there with her.