Baseless conspiracy theories about the killing of conservative activist Charlie Kirk have exploded on social media in the week since the shooting, with partisans from both sides pushing a dizzying array of claims at a speed rarely seen before, experts say. There have been more than 2 million unique posts on X, formerly Twitter, referencing conspiracy theories about Kirk’s killing, according to PeakMetrics, a nonpartisan data analytics firm. That’s roughly double the volume of posts using similar terms after the attempted assassination of then-candidate Donald Trump last year in Pennsylvania. The avalanche of unverified and unhinged claims from online influencers,...