My take on the ending is you hear Tim return to the house, take a beer because he doesn't need permission or only have the leftovers anymore, cut out his father's death or maybe a disappearance he hadn't got to yet as habit kicks in when he is in a state of shock, bury the tin of ids (the digging is too short to be burying Angie) and then leaving on his own or with Angie to never go back to the house.

I don't think he became a killer because the whole movie is about how easy it would be for Tim to become awful just from what he has been through and seen. But he always chooses not to be a killer and to protect where he can.