Seattle Man Admits Killing 48

Investigators had thought his killing finished in 1984. Little did they know that his last admitted murder was far more recent than this - 1998.
His first victim - who survived - was a first grade boy. Link to Seattle p-i story

How does someone manage to get to a stage of themselves where they can tolerate that kind of violence, and for such an extended period of time? How would you live with yourself? I don't understand it, and in a way, really don't want to.