Theories about The Guest [Spoilers]

Haven't seen much discussion about this but point me in the right direction if I missed it. I loved the ambiguity of "David" but I had a theory after the film was over. Could David actually have been their son, after plastic surgery, come home to take care of his family until his programming kicked in. Is there other theories? I am just confused why he would go to their house and "help" them otherwise.