Once they had gone to the house to murder her family Joe seems to have decided that he would ensure they were all dead, after Michelle fired at her father he then ensured that her father and mother were both finished off, whilst Michelle was running round not knowing what to do he moved the bodies (having to keep asking Michelle to help) and he then cleaned up the house enough so that when Scott, Erica, Nathan and Oliver arrived they would be able to get them into the house without too much suspicion,
Michelle seemed more interested in the deaths of Scott and Erica, and she shot them first, Joe again made sure he could finish them all off by asking Michelle where the other gun was as he obviously needed it to kill the remaining family, she told him it was in her fathers bedroom but when he looked it was her fathers gun and he knew that it would not be sufficient to kill them all so again asked her where there other gun was, he got it out of the kitchen cupboard and finished killing them all, it was he who took the phone off Erica and took out the battery as she tried to call 911,