He kills men because they upset or annoy him or make him feel inferior. In the case of the child, Bateman wished to see if he would enjoy it but found it unsatisfactory since the child's death would not affect as many people as an adult would. Periodically, he matter-of-factly confesses his crimes to his friends, co-workers, and even complete strangers ("I like to dissect girls, did you know I'm utterly insane") just to see if they are actually listening to him.