29 people were killed in two high profile mass killings in Dayton and El Paso. Our Eran Hami sat down with a criminal justice professor to learn the reasoning behind these gunmen. Virginia Beach, Gilroy Garlic Festival, El Paso, and Dayton. These are four sites of the most notable mass killings in the United States this year. Professor Tristan Kilgallon of Ohio Northern University teaches a class about serial killers and mass killings. He said there are different types of mass killers. The only thing they may have in common is rage or outrage toward somebody or something.