There are good teachers and bad teachers in every field...including the martial arts.
I am very fortunate to have a wonderful Taekwondo instructor. He's been doing TKD for nearly 30 years and he's very fair and cares about us as his students.
One example that comes to mind is that a few weeks ago, there were some teenaged punks loitering outside the gym where my school is located. I had seen these losers when I arrived for class that night and got that feeling that they were trouble. Apparently, my instructor walked into the gym awhile earlier and these a**holes started calling him names under their breaths, one gave him the finger, etc. This incident didn't seem to bother him, though I am sure he felt disrespected (he's a 5th degree black belt - TKD master). Well after we heard this story, one guy in our class, a white belt, wanted to go out and kick these punks' asses - half joking, half serious. Everyone else chimed in. Of course, my master can defend himself, but we're loyal to him and were ready to take the punks on if we had to.
My advice is to shop around for schools before committing to one. One of the important things is to observe how s/he teaches his or her class and their style and how you hit it off personality-wise with the instructor.
Danielle