Same as everybody else, really. Brain is complicated, and each and every one of us have their quirks - it's just that they are normally more simple and innocent.

Some people get born with paraphilias (i.e. zoo-, pedo-, necrophilia, nonconsensual sadism, exhibitionism etc.) and develop them naturally as teens, some others get something broken as a result of a traumatic experience. Science doesn't currently know exact mechanisms, except that it can be both natural and a result of trauma.

Now, the real question is - what factors should be combined with this to make people actually behave like that. Sleeping with kids/animals/dead people etc. is harmful and immoral, and they probably know it, too. Why do they do it anyway, and to what lenghts to they go to convince themselves it's worth it?