Dumbledore, in order to negotiate the vampires joining his order, gives Harry over (to Severus) not caring what harm would befall the teenager. Severus Snape's loyalty has always been to Harry, not to Dumbledore or the Order.
When he finds out that Harry is a horcrux and Dumbledore intends to have him killed, Severus goes to the Dark Lord with the information in an effort to save Harry's life, and in doing so changes the fate of the world. Harry Potter was the boy who lost too much and now he lost his mortality to save a life. Following the direction pointed by Dumbledore, he began the journey to find Carlisle Cullen, only to be distracted by a real-life Aphrodite he met on his way. I have a singularly impressive talent for messing up the plans of very powerful people - both good and evil. Being the Master of Death was a curse, plain and simple.
Try and find a way back to the future but have some fun dates in this fun adventure game.