why was hannah resentful of her grandpa will?

Because he shouted at the tv screen. He saw grainy pictures of WW2 and went all crazy shouting about what happened.

She hated when he would have one of his "fits" at a family party, which were often brought on by television coverage of the German Nazi's. She didn't understand his feelingg, and he never showed them outside of these irratinal fits of anger. They frightened her.