You see, most crazy people don't know they're crazy. Most.
But a few of them do. The ones that know they're demented, hide their dark intentions, because they know the ensuing asylum or jail time they will suffer if they do rape and kill that woman for no reason is not worth the fun they will have from acting out their deepest darkest desires. This is why the people that seemed "Normal" suddenly massacre and torture others. These people eventually stopped caring about the consequences, or just couldn't stand holding off the madness anymore.
You may be asking just how do I know how these monsters act? Well, I am one of them, and I am among you. I could be the bagger at the grocery store, that hot guy sitting across from you at the bar, or the teacher at your child's school. I hide in plain sight, and I have had much practice concealing what I am. You won't know until I strike, and then it will be far too late. All you can do when you are my prisoner is pray I will make your end quick. I won't make your end quick, I like to watch you scream in agony as I- I'll finish this later. It looks like you're about to wake up.