Brabantio vs. Othello

In the beginning, I was surprised at the situation between Brabantio and Othello. I understand that any father would be upset and angry if their daughter ran away and got married without telling anyone or getting permission. But, I thought Othello was the famous protector of the city of Venice? In those times, it seems as though one would be proud and happy if their daughter married the great hero. Everyone seems so power hungry, it's although it would be an honorable thing. Another aspect that I am confused about is Othello's nationality. The background information states that he is of African heritage. My misunderstanding is why he was made "hero" of the city, and how he got that noble position. Especially is those days, the separation between ethnicity was great.. so why is an African protecting the city of Venice?