First, he defied Poseidon when the sea god gave him a white bull to be sacrificed. He kept the white bull and sacrificed another, a substitute. Poseidon was so outraged that he cursed Minos' wife with zoophilia. She mated with the bull and gave birth to the Minotaur. The Minotaur was placed in the labyrinth to be trapped.
Second, when his son, Androgeos, was killed in Athens. He joined a contest hosted by Athens and won every event. Eventually, other contestants became jealous and killed him. Then, Minos declared war on Athens. As a peace agreement, Athens has to sacrifice seven young men and seven virgin maidens as victims to the Minotaur.