Why did you want to help her? she was some random stranger in a library and you, out of the blue, thought "i want to help this girl."
why? and what made you think she needs your help?
Deaf people have been treated by many peers as stupid. It's a fact. When I was in elementary school, I was one of the top performing students. If I had the highest grade on a test, believe it or not, my peers would tell the teacher I must have cheated by peeking at their answers.
In high school, I was paired up with a honour student for a debate in Advanced Economics class - he made such a huge scene about being paired up with the deaf girl, especially because he thought I would bring his grade average down.
Guess what? I got a higher grade than he did for the debate. People came up to me and high-fived me. They didn't do that to him.
so, when deaf people grow up around people who think they can't do well in school and obviously need help, it gets really old and tiresome fast.
You just need to try stepping into her shoes and imagine what her school life must have been like growing up, dealing with hearing people thinking that because she's deaf, she obviously must need help with her studies like she's not smart enough to learn subjects as well as if not better than them.
You got upset because your ego took precedence over trying to understand it from her point of view.