How do we know that the guy wasn't just drunk, stupid, or otherwise mistaken about what he was doing?

I once unlocked a car that I thought was mine, put the key in the ignition, and started driving away. Then I noticed that there were objects in the car that I didn't remember putting there, and it smelled different. Then I thought someone had broken into my car and used it and trashed the inside. Then I thought about calling the police. I pulled over, looked the car over, scratched my head, and notice an identical car in the same parking lot.

I had just driven someone else's car and it took way too long for me to realize it.

I also didn't know it was possible that your key could work on any other vehicle.

Just sayin'. He could have just had his head up his butt.