After spectacular acts of violence, the hunt for suspects’ online trails — and the sources of their presumptive radicalization — often begins before a name has even been officially confirmed. Sometimes, as in the case of the 23-year-old mass shooter who targeted a Minneapolis Catholic school in August, posts provide evidence of nihilistic ideologies or other motivations (in that instance, an apparent idolization of other mass shooters and a desire to perform for aspiring ones). In other cases, such as that of alleged United Healthcare CEO assassin Luigi Mangione, the evidence is confounding, revealing an online profile that nobody would...