Just before Tyler Robinson turned himself in for the murder of conservative activist Charlie Kirk, authorities say he appeared to leave a trail of incriminating messages on the online gaming platform Discord. At first, his messages were playful. When a friend on a group chat noticed his likeness to the skinny white man in the grainy photos released by the FBI of the Utah Valley University shooting suspect — asking Robinson “wya,” an abbreviation of “where you at?” — Robinson was quick to joke: “My doppelganger is trying to get me in trouble.” But in a later Discord chat, Robinson...