Mr. Stelling is not really a bad person, but he ends up causing a lot of people problems. Like so many characters in this book, good people often cause a lot of problems and take bad actions because of ignorance, stubbornness, pride, etc.
Mr. Stelling is very set in his ways. He is confident that his way of teaching and his rather old-fashioned view of education is correct, and he is going to stick with it come hell or high water. This is rather ironic, given that he is teaching Tom, who is himself confident to the point of arrogance. Also like Tom, Mr. Stelling sticks to his first impressions. He feels that Tom is dumb since Tom has trouble learning with Mr. Stelling. And Mr. Stelling never really alters that opinion of Tom, or tries anything different to reach him as a teacher. Mr. Stelling is also a bit full of himself, which impacts him negatively as a teacher:
Mr. Stelling’s duty was to teach the lad in the only right way – indeed, he knew no other: he had not wasted his time in the acquirement of anything abnormal. (2.1.9)
Basically, Mr. Stelling refuses to change his ways for anyone and, as a result, is not a very good teacher to Tom. Overall he represents a bad sort of education system that the narrator comments on extensively.