I think the end of kills was basically Myers in survival mode to survive the mob and that weakened him severely, he was shot, beaten, stabbed near the spine etc, he summed up enough strength to take them down and then Karen, but after that he went into hiding as he was spent. Considering how he is in this it seems to have weakened him to the point of being near useless, he wasn't the same Myers in this as evidenced by his confrontations with Corey and the fight with Lorie, he seemed a lot more vulnerable and not even remotely as resiliant. His only Myers-esque kill was to the small girl the doctor was trying to bone, which isn't saying much.
It was a bit weird how he seemed to basically "hulk up" during his encounter with the cop, he was stumbling about at the start of the fight and got stronger as he stabbed him and killed him, but even then he was nowhere neaar full strength, if he had been Corey would have never been able to put him to the ground and take the mask. Just seems age and injuries sustained caught up with him in the end.