Two things. If Michael/Felos succeed in killing Terri by starvation/dehydration they have an end that is quite chilling to their book and movie. I mean what makes a better end to a scary movie than genocide.
If they don't Michael has to take care of her. No wonder he is in a hurry, he has embezzeled her funds with Greer and Felos help, now he would be having to work to take care of her. That would be a switch for him, and who would want to see him in their doctors office or hospital?
If they don't Michael has to take care of her.
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No... Terri's parents will take care of her !!!