If Parker was possessing Josh throughout the whole movie, why didn't he just kill Josh's family?

As far as I'm aware, the movie didn't give a neat explanation for this, but from what we do know, it was always Parker's mother driving him to kill, and although Parker clearly enjoyed killing, we have to assume he wanted to continue his killing spree 'on the quiet' like before, just under the identity of Josh this time. Killing the family would only attract attention and make it difficult for him to stay in possession of Josh.

This is at least backed up by the first bit of dialogue we hear from Parker (via the possessed Josh):

I wanted to stay so much longer... the feeling of being alive again. You cannot comprehend how good it feels. To breathe. To eat. All the little things that you brush off, like a fly. But she wanted me to kill you. She wouldn't stop. Wouldn't stop pushing me to do it.

As far as I think, Parker and the other dead's desire was only to possess alive people's bodies, escape hell and live again. But when everyone got to know that Josh was in fact possessed by Parker, Parker didn't find any other solution to keep him alive again only to kill anyone who makes an obstacle in his way.