The Headmaster of Hogwarts

Severus Snape, after killing the greatest Headmaster Hogwarts has ever seen, returns to his beloved school to claim it in the name of Lord Voldemort. One-shot. (Why did the gargoyle guarding Dumbledore's office jump aside when Harry said 'Dumbledore' to enter it? Because that was the password. Find out who set it that way, and why.)