I've said this before, but it could be a simple CYA. His daughter was kidnapped. You think he wanted to tell Patsy and the world that he didn't check the doors? Not only did he not arm the alarm but he didn't check the doors.
I agree. I've run up against this line of thinking before, and my response is this:
1) In order for the Rs' story to work, they HAD (HAD, HAD, HAD) to find the RN FIRST before they knew that anything else was wrong. Nothing could seem out of place at first. They couldn't claim that JR woke up and said," Gosh, it's cold in here."
2) The Rs had to give the impression that they were being completely responsible in terms of personal safety, or as responsible as they could be. Claiming that you'd left the door unlocked would, in retrospect, strike most police as stupid, which would take away from the sheer tragedy of this killing. And that's the key. It had to look like their best efforts to protect their children were not enough.
3) Just as an add-on, it's worth mentioning that the Rs initially tried to direct the police toward people who would not have needed to break in because they had easy access.