I'm trying to flesh out a character in my book that is basically a "town rowdy" that takes sport in antagonizing people, especially the main character. At this point I'm having a hard time making him believable in my own mind. I know part of it is just the nature of that person, but I want him to be more than just a convenient source of conflict. Is there any compelling reason why someone would just wander about, making trouble for the main character? Or any character in general?
Unhappiness, frustration, powerlessness at home or in life. Some bullies are sexually excited by power. Others enjoy hurting people as a kind of revenge against someone or some situation hurting them, against which they have no easy way to retaliate: It is easier to shove Sheldon on his ass and make him cry, to take back some of what the bully's father took from him by slapping him in the head and calling him a worthless idiot that should never have been born.
Depending on the age, many bullies think aggressiveness makes them more attractive, and often this tough guy in charge is IRL rewarded by women with sexual favors that want to be the boss's wife. Or girlfriend, or just want to be seen as his protected property: The queen to his king. That can create a positive feedback loop for the bully, the more he bullies, the more women in his harem.