For example, Dylann Roof googled "black on white crime" after the Trayvon Martin incident. He read all these stories and comments about the crimes that blacks committed against whites. He grew angry and vengeful. It was messing with his mind. In his mind, all blacks were criminals, evil, dangerous, etc and must be dealt with. He wanted to start a race war.
Maybe if he also googled "white on black crime" and read all those articles, then all that info might have counteracted his beliefs about blacks. He could have watched documentaries such as "The Ku Klux Klan: A Secret History" and seen all the vile things whites committed against blacks for decades.
He would have then seen that there's good and bad in all races. Maybe he already knew that but just didn't care and was being a hypocrite.