On Saturday afternoon, Thomas Crooks, 20, climbed onto the roof of a building in Butler, Pa., carrying an AR-style rifle purchased by his father more than a decade ago and loaded with ammunition Crooks bought that morning. People in the audience spotted him and pointed him out to law enforcement, but Crooks managed to open fire at former president Donald Trump before he could be apprehended. He was killed shortly afterward by a Secret Service sniper.It’s not clear why Crooks did what he did. He was a registered Republican but appears to have made a small donation to an organization...