A person does not usually go from normal acceptable behavior to the state of monster in one big jump. Deviant behavior happens in such little steps that one gets acclimated to each phase of deviance before moving to the next, and so on, and so on.  At each step in the transformation to monster, what ever gratification that one achieved at each step dulls, therefore the stimuli needs to be increased to get the same result.  Someone, Like Ted Bundy who happened to be per-disposed to be a serial killer, started off with cartoon like pictures of women being abused, soon this wore thin so he took it to the next level and plunged deeper and deeper into aberrant pornography.  When this quite stimulating him, he tried physically abusing women, when this wore off he started killing them and having sex with the dead bodies.  No matter what his per-disposition, Bundy, at the beginning would not have every thought that he could be the monster he became.  I am pretty convinced that most people are capable of any evil if that evil is realized in small enough steps.  1 Peter 5:8 warms us of this capability - Be sober, be vigilant; because your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour.