Jack Sparrow became a pirate because he refused to sell his shipmate to slavery!
Although it's not mentioned in any of the movies, there was a series of books that detailed the backstory of Captain Jack Sparrow. Turns out that Jack Sparrow was at one point a legitimate first mate of an East India Trading Company ship.
While he was in Africa, Cutler Beckett makes him captain of a ship and gives him an assignment. He's to go with a girl name Ayisha to her island of Zerzura and steal their treasure and sell the natives to slavery.
The idea of that eventually is too much for Captain Sparrow, who decides to disobey the order and because of that, he became the pirate that we know and love from the movies.