I'm probably simplifying things a lot here, but I agree with Spleen that a person doesn't necessarily need to be 'mad' to think ugly hateful thoughts or to do violent or indeed murderous things.

Take the guns away and the shock factor (I'm sorry to put it like that) of mass killing is removed.

However the person in question would still be the same. Instead of being someone who shot numerous people, he'd be someone who beat up a lone black person in a darkened ally, or maybe at best attacked an individual black person with a knife.

No more or less nasty or racist, no more or less mad, just without the power that guns have to enable nasty, hateful, violent people to inflict such huge damage.