I had a student who enjoyed tormenting others. It was awful. He'd watch to see what kids' triggers were, and then do precisely whatever it was to set them off. I knew this boy for almost two years and nothing ever made it better, and over time, he got worse. He loved it. Take a kid's beloved stuffed Barney and enjoy the screaming, shove a kid to the cement and smile. There was no reason for any of it; all of these instances were unprovoked. Talking about feelings, time-outs, calling Mom and Dad, it made no impact. He was a predator, in kindergarten form.
Then one day, we got a transfer of a boy with high-functioning autism. Things had to be just so or he'd have a meltdown. Little Predator did what he did best and started preying on him. We stopped it as often as we could, but in a packed classroom, he still managed to get around us. And he'd take the kid's pencil. Nudge his toy. Scribble on his art project. Hiss a name. And then the victim would have his meltdown, throwing things and screaming and smacking other people, and Little Predator sat back with a grin.
Finally one day, as I let the kids out for recess, Little Predator did or said something to this kid (it was always unprovoked) and the kid lost his temper and punched him in the face. Little Predator looked back at me and another teacher, waiting for us to haul that kid off the playground, and we pretended we hadn't seen anything. And we didn't do anything. Little Predator deserved it. And he deserved every single punch from other kids that I'd managed to stop in time. And you know what? After that punch, Little Predator didn't bother that kid any more.