Rutger Hauer's android leader in Blade Runner and Gene Hackman's Sheriff Little Bill Daggett in Unforgiven provide parallel indelibility. Look at Jeffrey Dahmer. They had parents. They had a life. They rationalize like you and me.
All villainy is motivated.
Assuming the environmental forces are in place, to understand the villain's mens rea, the screenwriter's guide to villainy might be viewed from one of two angles. Given the proper nurturing, we are all capable of anything. To understand others' villainy, therefore, we need only look into ourselves at our weakest, most enraged, or most desperate and vengeful moments. In Mommy Dearest, Joan Crawford bashed away. But, we understood why.