Alex Jones, the conspiracy trafficker who runs the website Infowars, believes that Sept. 11 was an “inside job” and that the massacre of children at Sandy Hook was faked. These are cruel falsehoods that most people don’t want to confront on broadcast television. Megyn Kelly’s decision to interview him, for a show to air Sunday night, has been roundly criticized by people who suggest that we would be better off denying this fringe extremist exposure.

Indeed, when Mr. Jones was merely a marginal figure on the paranoid right, the case could plausibly be made that he was better left in obscurity. But now that, at least according to Mr. Jones, the president of the United States has praised him and thanked him for the role he played in his election victory, it’s too late to make that argument. We can’t keep ignoring the fringe. We have to expose it.

We would naturally prefer not to reckon with the worst of what people do or say on the margins, but we have to. Especially if it seems possible to trace a line from vicious rhetoric on a computer screen to violent action. We can’t know exactly what drove a man to open fire last week at a field where Republican members of Congress were prepping for a charity baseball game, but in the aftermath of that shocking event, we can trace the shooter’s online presence to the fringe world on the left he inhabited where he railed that “It’s Time to Destroy Trump & Co.”

While Alex Jones has no exact analogue on the left, we have to watch him, and also watch out to make sure that something similar is not emerging on the left.