Didn't the father find...
...putting his dead daughter to bed rather gross? As well as combing her matted hair in the bath? The way she was white as a sheet, with wonky eyes and the great big staples holding her skull together, made for an extremely unsettling image. I mean, the father desperately wanted his daughter back, but even if she was going to be like this from then on? Not to mention a change in personality? I can only put it down to the evil force in the sematary exerting its will on him to join the undead, along with the rest of his family.