It's possible he snapped and didn't have his cover-up quite figured out. But, I can't help but wonder if it is also possible if everything he did made perfect sense to "him"?
I have recently been going over the Scott Peterson case again. Peterson appeared to be an intelligent man. He had a college degree, so he had to be of at least average intelligence. Initially, there was very little evidence that a murder had even occurred, but his behavior was so "off" that police suspected they were dealing with a first degree murder case and that he was responsible. Even though it was clear the murder had been planned, Peterson still made mistakes. But ultimately, I think it was his extreme arrogance that was his undoing. He even continued to call his girlfriend after his wife disappeared to try to continue that relationship!
It just boggles the mind, but apparently to him, it all made perfect sense.