You never truly know someone - we don't even always know ourselves.
Life is a long journey. People are going to make mistakes. Sometimes people do bad things.
Noone sane is the bad guy of their own story; and bad people certainly don't come with horns on their head as standard.
In that sense then, you have to allow yourself to be vulnerable to be a good person - you have to take the risk to let people into your bubble.
And sometimes, they burst that bubble.
The hardest thing to do is not let it change who you are - and keep being a good person. But it's the right thing to do.