If they moved the bodies to the back bedroom and locked the door behind them (per Bobby), they didn't want anyone to come up to the front door and see the bodies inside. At night, especially, anyone coming to the door, finding it locked and seeing no one in the living room would assume they were not home or in bed.
This may mean the killers struck here first and wanted to make sure no one checking on CR1 and Gary raised the alarm before the killers were finished at the other crime scenes and gone.