There are numerous reasons seemingly normal people can commit heinous crimes. They can be overwhelmed by a powerful emotion, or they can be devoid of emotion entirely. Psychopaths are often quite normal people with one particular abnormality - the absence of conscience. You know that sense of visceral disgust/horror you get when you think about someone committing murder? What if you didn't feel that? What if instead, the thought gave you a rush and made you curious to know what it feels like? What if you approached the concept of murder as nonchalantly as picking up groceries from the store? That is how the mind of a psychopath generally operates. It's not this glaring insanity that drastically changes the way they operate in most situations. It's something subtle which you might never notice observing the person from the outside. I am purely speculating, but from what I have learned about BK and the murders, I suspect he is a psychopath.