Harry Potter and the Goblet of Fire question?

Throughout the books, Snape is on strict orders from Dumbledore to appear to be a devoted follower of Voldemort. It's because of this reason that Snape kills Dumbledore in the sixth book, and why Snape betrays the true date that the Order of the Phoenix decides to transfer Harry from number 4 Privet Drive to the Burrow in the seventh book. So for this reason, Snape had to go to the graveyard to prove to Voldemort that he is still a "true" death eater.