User blog comment:B214/Question and Answers 3/@comment-75.93.25.66-20140719221006/@comment-75.105.131.164-20140719224129

Azazel has not said why he hates him. However he is the one responsible for Vali's father tormenting Vali to the point Vali ranaway. Since Azazel raised Vali I would bet this is the main reason.