A white-supremacist bigot who killed 10 people in Buffalo had been posting for a long time about his plans for killing.

Visible signs that someone is planning violence are generally not certain or clear, especially when you don't know very well the person that is giving the signs. Chances are most of the people who might strike someone as suspect would never carry out any violence. Given that situation, what could possibly be useful to actually do? Nothing, I think.