A person who is kind and vulnerable may have a basic value that needs to be challenged. Your daughter may have internalized the idea that the way for her to be a "good" person is to go through life assuming that others are good, unless they prove otherwise. That's dangerous, and it's backwards.

Call to her attention some of the stuff that's on the news that reveals how evil humans can be. Teach her that, until someone demonstrates that they are caring and honest, she should not assume anything about them. Any random person standing next to you at a bus stop can possibly be a viscious monster. Children as young as 5 need to be taught that.

It's appropriate to be courteous with any stranger. That doesn't mean opening yourself up. Even with people you don't like and don't trust, it's appropriate to be courteous when circumstances require you to interact with them. With people who you know to be bad, there is little to be gained by telling them you don't like them. It's fine to keep negative opinions quietly to herself. To avoid getting in fights is smart. I know that you know all these things. Kids need to be explicitly taught. Your daughter needs to carry a "shield" with her. Another word is "boundaries."

It's a shame that, even within the sanctuary of your home, wicked people can gain entrance and start pulling crap. That's the world we live in. Your daughter can still be a kind person, but doesn't have to be a foolish person.
Too much innocence is a luxury we can't provide kids today.