Name some reasons why Bruno dislikes Lieutenant Kotler.

One reason is that Lieutenant Kotler is incredibly patronizing towards Bruno, always calling him "little man." Children of all ages hate being made to feel like little kids, and Bruno is no different. Kotler, the blond, blue-eyed Aryan, is a popular figure in Bruno's house. But not with Bruno. He sees past Kotler's clean-cut exterior to behold the evil that lies beneath. Bruno has always had a bad feeling about Kotler, even as his sister Gretel had developed a crush on him. But Bruno knows the truth about Kotler, how incredibly cruel he can be. One day Kotler sees the young boy reading Treasure Island and snatches it from him in an attempt to goad him into losing his temper. Yet this is nothing compared to how he treats Jewish prisoners in the concentration camp, culminating in his murder of Pavel for the "crime" of spilling wine.
Bruno still sees the world through the eyes of a child, and so his outlook on life remains somewhat naive. Ironically, however, this allows him to see Kotler for what he really is: a bully, the biggest bully in the playground, so to speak. And it is this child's-eye view that provides Bruno with a much greater degree of insight into Kotler's character than the adults in his life.