There are some teachers who shouldn't be in their profession. I know how awful some students can be, I've been at a state school for five years, but some have a complete lack of enthusiasm for their jobs. Yes, it's a tough job and the pay isn't great, but teachers should only go into that job if they are able to deal with it.
In year 11 for science I had a dreadful teacher. She said to the class at the beginning of the year, somewhere along the lines of "You know what? I don't f***ing care if you fail your GCSEs, I have mine anyway". Eurgh! The majority of the class I was in were a bunch of chavs, but in the end I was so annoyed with the woman I had to teach it to myself. Students and teachers like her are very infuriating, because it deprives the students who want to learn. I was utterly stupid to not accept going to a private school when my parents offered it.
I used to feel sorry for the teachers when I was in year 8/9, when suddenly everyone was report, having fights and they behaved so poorly. It really is quite shocking that some children were brought up so badly. Their parents had a complete lack of respect for the teachers too. Though saying that, a teacher should be able to control a class and I do believe that there should be stricter disclipine in schools.
In answer to your question, yes some teachers do go into teaching for the 'wrong reasons', like the teacher I mentioned above. However, there are some teachers who really are hindered from performing well simply because students misbehave to such an extent.