OK, so it's pretty easy to list all the reasons that the Mayor, the play's antagonist, is a jerk. From the very beginning, he seems unlikable. He doesn't want to take part in the party going on at Dr. Stockmann's house. In addition to being standoffish and easily offended, he's judgmental of his brother, Dr. Stockmann. The Mayor also makes a snide remark about Hovstad, revealing himself to be prejudiced against people of a lower-class origin.
The Mayor's bad side is further revealed later on in the play. When he receives the report from the Dr. Stockmann about the contamination of the Baths, he does everything he can to cover up the bad news. Mayor Stockmann has several reasons for doing this. The chief reason probably being that it's all his fault that the Baths are contaminated. He went against his brother's guidelines when laying the Baths' pipes and now the town's big investment is in peril. The Mayor seems much more concerned with his own public standing, than with the safety of the people using the Baths. Throughout the rest of the play, Mayor Stockmann mercilessly tries to ruin his brother's life in order to keep the truth from being revealed. He never really shows one shred of guilt for the fact that he's totally messing up the life of a family member.
Even though the jerk factor is pretty high with the Mayor, he's not a totally one-dimensional character. Early on in the play, the Doctor even defends his brother's nasty disposition saying, "We must remember that Peter is a lonely man, poor chap. He has no home comforts of any kind; nothing but everlasting business" (1.124). So, maybe the Mayor isn't grumpy with people just because he enjoys it. Maybe, he's just a lonely and misunderstood.
Mayor Stockmann is also a lot more practical than his idealistic brother, who doesn't take into consideration the fact that the proposed renovations will destroy the town's economy. It's also important to note that although the Baths were the Doctor's idea, it took the Mayor skills to get them built. (Too bad they were built the wrong way.)
The Mayor's suspicion of his brother's findings isn't totally close minded either. Dr. Stockmann does have a tendency to come up with wild ideas, and people didn't know a whole lot about bacteria back then. To be fair, the Mayor does have some room for skepticism, especially when it'll cost the town he's in charge of so much trouble. So, what do you think? Is Mayor Stockmann a pompous jerk or just misunderstood?