Antifa’s profile is rising so quickly that it even got name-checked at last week’s presidential debate. As the movement enters the mainstream, its sympathizers are escalating the violence from throwing rocks and blocking doors to physical assaults and, for the first time, killing. The rising level of violence from the group marks a sinister evolution from minor street crime to guerrilla tactics usually reserved for revolutionaries. “The fear is that we are witnessing the beginning of something that could be more violent over time,” said Michael Kenney, a University of Pittsburgh professor who has interviewed Antifa sympathizers. The November presidential...