I have been studying the why's (motivations) of both murderers and sexual-murderers( not including murders which are unintentional, crimes of passion, gang related, or for hire). The why's re incredibly complex, and can be broken down into a combination of factors: formative development, predispositional factors such as trauma, genetics (e.g. extra Y chromosome), reliance on early development of fantasies as opposed to object relations to achieve mastery of conflicts , paraphilia development (sexual deviations and fetishes), exposure to violence as child, poor self esteem and weak self image, and, the use of autoerotic activities/masturbation to deal with anxiety. There are more.