Why didn't William just use the scanner on her?
I mean there was a scanner laying on the ground he could have easily picked up and used on his daughter to confirm if she was a host or not, instead of going ahead and killing her and confirming after she was dead by cutting her arm open.
I mean its a 50/50 chance she could be a host or your actual daughter, why the fuck would you kill her instead of just scanning her to make sure?