You left it in gear knowing it will move on its own on a flat surface, and you think someone jumped in, hit the gas pedal and went for a ride just for fun? Do that again, but stand there and watch it do it again all by itself and you'll have your answer. If anyone would have been brave enough to risk injury by jumping into a moving car, they were trying to hit the brakes. I've done it before with my cars, and you do not want to get hit by a moving open door.

By the way, don't really stand by the side and watch it crash again. Shift into reverse and sit in the car, then see what happens when you take your foot off the brake pedal.