Why do people get mad when you catch them doing a bad thing and you get upset about it?

I recently found out that my ex-girlfriend (whom i've known for about 12 years) and an ex-friend who was like a brother (whom i've also known for about 12 years) dated when we were all still cool and they also had . I found about about from a mutual best friend of theirs. He came up in a conversation between her and myself after a few weeks of us trying to work things out and get back together. Then, when I bring up the fact that I knew about them having she just came out and admitted that they DID have and were in a relationship. I wasn't quite as mad before because the information didn't come from the source so to believe that it's true is foolish, but when she told me i was pretty upset because I actually loved this girl, and he was like a brother to me. So for them to go behind my back knowing what type of relationship i had with both was un-trustworthy, un-friendlike and just distasteful. I'm still civil with her, but that's where it ends and she's upset because she feels like she didn't do anything wrong, especially when she's had feelings for him since the same period of time I had feelings for her. Funny thing is that HE is the one that hooked her and I up. SHE knew we were real good friends. Knowing this information, why did she get with me while she liked him in the first place? SHEESH...