Humans are just really advanced pattern recognition machines, when things are nice and easy we find them pleasing. That's why symmetrical faces are universally attractive. Our brains automatically adjust faces to be more symmetrical, so a face that's already symmetrical is just easy mode.

So when something bad happens, we search for a cause. If we can't find one, it stresses us the fuck out.

Which leads to blaming "easy" scapegoats. People that appear different or are already disliked.

It backs up your belief that person is evil, and makes you feel like you're preventing future bad events. Our brains are just wired to feel good about a solution being found, it doesn't care if the solution is right.

What's even worse is if someone tried to warn the group, was ignored, and then proven right. They're almost always labeled the scapegoat or else the people who refused to heed the warning look bad.