The Pennsylvania man accused of decapitating his father undoubtedly spent time in dark places on the internet. That became abundantly clear when news broke that the man, 32-year-old Justin Mohn, had been arrested after holding up a severed head in a YouTube video and calling for the execution of federal government employees, starting with his own father. In his ramblings, Mohn called on “patriots and militia members” to win their country back from “the globalist, communist takeover of America.” It’s hard to get to the point of making manifestos on YouTube (let alone broadcasting lethal violence online) without some exposure...