This is a good one. I think it plays into how we view violence and crime in the US. I would also say that most people fit into several groups/labels, so often even if someone from a person's group is doing something that would be frowned upon, they make sure to identify that person by another group instead of the one they identify with.

I think a big part of this is that people in general are scared (or at least feel uneasy) with differences, and feel safe and comfortable with similarities. When someone does something they strongly disagree with, they feel a need to make that person different from their group. They don't want to believe that they (and their circle) are capable of whatever negative thing was done. They need to keep their group as a safe haven for themselves.

There's also comfort to be found in tricking yourself into believing that dangerous people can be identified ahead of time using superficial traits. It's less scary to people to think that either they themselves, or LEOs/gov't can identify the dangerous people and take proactive measures before anything happens, than coming to terms with the fact that violence/crime (or even just unappreciated behaviors) are part of human behavior and can literally be perpetrated by anyone at any time.

Of course there are some correlations, but even these tend to be pinned on one common trait instead of delving deeper and really finding out what is causing certain actions. I can't think of one group of people based on one shared trait, where the vast majority aren't violent/criminal/malevolent.

I think ideally people would realize that A) it is nearly impossible to identify who is a threat ahead of time on a broad scale (even on an individual level it isn't that easy), and B) 99%+ of the people you come across in your life are zero threat to you.