The more true crime stories I read, the more I believe that criminals are dumb. We watch a lot of TV shows describing elaborately conceived murders and coverups, and yet in real life, most criminals are rather obvious and clumsy. Some smarter than others, some less clumsy than others, but generally not terribly clever. I think when you and I sit here, as we're doing, in our "armchairs", it's easy to think "oh wow, how stupid, i would never do that".... but imagine yourself in an altered state, where you're amped up on adrenaline and trying to figure out how you're going to get away with something, and the clock is ticking. You can conceal things in and around your home much easier than you can anywhere else, because there are no cell pings or cameras to track you to an unusual location (although neighbors security cameras are often the downfall in some of these cases).
So I guess what I'm saying is that they're not of above average intelligence, and other than in actual psychopaths, they have similar stress responses to the average person, and don't think particularly clearly when under the stress of concealing a crime.