His height and stature have always bothered me. You know how sometimes you wish you could will into being your thoughts? There is this part of me that wishes with everything I have I could go back and tell her not to go for that run or somehow change the outcome which leads me to this: he is such a pipsqueak (I remember the first time I saw him at the SO after his arrest, it stuck with me how tiny he was) and she was young, had a physically fit solid body, was athletic, etc. It doesn't make sense she wouldn't have been able to overpower him unless it was a surprise attack. I think the wound that penetrated her skull was the first, a disabling attack from behind that he was somehow able to pull off. That's why there was only so much blood in the trunk, it was the only wound at the time. It's also why he saw her twitch when he opened it, she was still alive. The rest all happened in the cornfield.