What caused a person to become a mass killer?

And what gives someone an evil nature?

The limbic system of the brain takes control .

Over rides the neo-cortex of the brain . Involved in higher order of conscious thought , amoung other higher forms of things . Such as sensory perception , motor commands , spatial reasoning , language.

Its not evil so much , its just the primative part of the brain that takes control of ones behaviour , conscious thought , etc. And awareness of ones actions .