It's the sequence of events we call "life".
Everyone has light and dark within in them and everyone has an innate predisposition for what I could only simply describe as "white", "grey" or "black". Most people fall under the grey and tend to "step in/closer" to the one of the others according to the events that take place in their life and their environment.
Of course, life takes us through both good and bad so people change... we are dynamic beings constantly going through not just physical evolution but emotional/mental evolution too. Sometimes, that evolution isn't always good and can lead us to the "black" and life doesn't create a path back for a long time, if ever.
With that in mind, some people have the innate predisposition of "black" and therefore are naturally comfortable/accustomed to causing damage. Exceptional events and circumstances may see them make fleeting visits to "grey" or even "white" (remorse, protection of loved ones) but since they have a predisposition, they are very sensitive towards circumstances which can take them back to their comfort zone.
TL;DR: It's human nature.