We had a maths teacher in high school who was known for being "tough to work with". I ended up in his class. He would belittle people for getting answers wrong. He would put down people for the way they dressed or even the words they used. It got to the point in which he would call kids to the blackboard to solve problems he KNEW were too advanced and then make them cry with a mix of personal info and just the average "you will never amount to anything."

One situation that sticks out was with a guy who he called up to the blackboard who just didn't get the current topic. When he couldn't solve the topic fast enough the teacher would interrupt his writing, would mock him on being slow. Then he said "it's no wonder [classmate] broke up with you. You must be a bore in conversations if you can't even solve this." he kept going, just being purposefully mean. The guy ended up crying and the entire class was silent.

The funny thing is that as soon as he wasn't standing in front of a class, the teacher was the sweetest, kindest dude. I once approached him because I was behind on the lessons due to being sick and he just handed me the folder that contained everything for the entire year. I am not sure what caused this Dr. Jekyll and Mr. Hyde thing but it was utterly baffling.