Girl who liked me was very insecure?

A girl who liked me was getting very insecure when I wasn't showing interest in her. After a while she started to get angry at me. I didn't have any feelings for her because I saw her as more of a sister figure, not necessarily because I didn't find her attractive. She even wore more make-up than usual one day. I wrote her a two page later explaining why I wasn't talking to her. After she read it, about 4 days later I went to sit with her at a bench. Ready to give another reason I didn't want to be with her, wanting to say I was saving myself for marriage, and was afraid that she wasn't willing to wait. As I began to talk, she instead told me that she was in a relationship, and that she acts the same way around everybody. She was very blunt about it, making it obvious she didn't want to get rejected first, so decided to make an excuse.

Am I the one at fault here? Was it so bad that I didn't want to be with her? Why did she take it the wrong way?