I don't think that prejudice is necessarily a bad thing. Quite literally, the term means pre-judging. Forming a gut reaction opinion, usually based on the stories of our culture or by our own past experiences is a survival tool. It allowed us to stay alive long enough fr all of us to be born.
Prejudice obviously has a dark side, though. When you judge people or situations with no actual knowledge about what you are facing, then you are likely to make really terrible decisions. Stereotypes may exist for a reason, but they are not always true. And no matter how similar two situations may be, they will never be identical.
In the modern world, we rarely need to make split-second decisions. A calmer, rational mind is a more effective one. We should always get as much information as we can before settling on an opinion or taking any actions.
The thing with pre-judging is that you can't stop it. The mind makes judgements. It makes harsh ones and usually makes you work hard to overcome them. (Whatever you go in expecting, you will almost surely find.) So I can't blame people for doing it, but I can blame them for acting upon their prejudice. If you don't know for sure relevant specifics about the person or situation, then you have no right to make assumptions and base your decisions and actions on those assumptions.