The wrong kind
They're out there. And a lot of them appear normal, especially if they're good at covering it up or manipulating those around them. If you've never run into them before, you can easily be taken in, especially if you're kind and hospitable.
Had one in my life. Was schitzophrenic. Took four years to completely uncover the problem. Saw a few warning signs but the guy was a long time NASA engineer with TS clearances. And it got ugly, very ugly. I was lucky to escape with my brain still intact.