This is regarding "The Boy in the Striped Pajamas." Why is lieutenant Kotler so cruel?

Lieutenant Kotler is a Nazi guard in "The Boy in the Striped Pajamas." Despite his relatively young age, he is zealous and takes delight in being cruel to the prisoners who are forced to work as servants in Bruno's home. While Kotler's past is not explicitly discussed, parallels are drawn between his behavior and Gretel's enthusiasm for the Hitler Youth propaganda being fed to her through the tutor she shares with Bruno. It is feasible to believe that Kotler was indoctrinated into the Nazi agenda, just as Gretel is being indoctrinated. Another comparison is drawn between Gretel and Bruno, who resists Nazi programming despite the fact that he was raised with it. In a basic sense, Kotler is cruel largely because he wants to be, while Bruno and Pavel remain kind and open-minded despite their circumstances.
Another reason for Kotler's abject cruelty is his interest in impressing Gretel as well as Bruno's mother. Despite the fact that Gretel is only 12 and he is 19, Kotler returns her crush to a degree. In an attempt to impress Gretel and the children's mother, Kotler is unnecessarily cruel in his dealings with the prisoners. He clearly does not view the servants as human and takes every opportunity to exploit, beat and persecute them to further his own career and improve his standing among the Nazi guard. 
Kotler's cruelty is not limited to other human beings. On page 172, Bruno reports witnessing the young soldier shooting a dog for no apparent reason other than the fact that its barking annoyed him. These combined actions lead to the conclusion that Kotler is generally cruel for the sake of being cruel as well as to advance his reputation in a group that rewarded such behavior.