After a string of shootings, bombings and assassination plots in the US, investigators across the country have run into the same frustrating problem: they can’t pin a clear cause on some of the attackers. These aren’t easy cases to sort into the usual boxes. The suspects aren’t clearly aligned with Democrats or Republicans. They aren’t claiming jihad, waving white supremacist symbols as their central creed, or pledging loyalty to a named group. In their writings, what shows up instead is something broader — disgust with society itself. US federal prosecutors have started using a new phrase for it: “nihilistic violent...