Its just a little detail in the story. But it almost breaks my heart that Snape took Dumbledores name as password for the Headmasters-his own-office. Personally I dont believe Snape liked Dumbledore that much. But I really believe he respected him deeply as great wizard and his guide in the fight against Darkness. It is hard and very painful to imagine how it must have Severus hurt inside to kill Dumbledore and therefore destroy any trust anybody else on the world in him. I am very sure his soul broke in two because he had to act like a Death Eater again, something he hoped to be past for him forever: Cruel, merciless. There was no side left to go for him.