Snape is possibly the most complex character in the whole Harry Potter Universe.  He is sarcastic and controlled and makes no secret of his dislike for Harry because of his resemblance to his father who apparently bullied Snape when they were at Hogwarts together. However, it is Snapes love for Harrys mother Lily, that leads to him rejecting the dark arts and becoming a double agent for Dumbledore. Snape is ultimately killed by Voldemort but earns forgiveness and compassion from Harry once his back story is revealed.