Mr. Keller called Annie a tyrant because she pushed Helen, something that had never been done before.  Annie pushed Helen, not out of spite or meanness, but a genuine concern for her well being.  The Kellers, on the other hand, felt that it was better and easier (for them) to simply allow Helen to do as she pleased, which of course, was great for Helen, who loved that idea, until Annie came along.  Annie was the first to ever challenge her or make her work for what she wanted or needed; because of this, Mr. Keller felt that she was too harsh with his daughter.

Keller calls Annie a tyrant because she is demanding what she needs and Keller is not used to people in his life doing anything other than what he wishes. It is ironic that he would call Annie a tyrant, since Keller is the true tyrant. Annie is eventually able to win his respect by the end of the play.