"What motivates someone to do these horrible things????"
You haven't been keeping up.
Teaching White Privilege in schools causes this,
Black Lives Matter causes this.
Nation of Islam leader Louis Farrakhan telling black people "rise up" and "stalk and kill" those who are "killing us,"
Unless we fight these new anti American movements the killing will only increase.
Being ignorant of whats going on will only increase the killings.