The Half-Blood Prince of Slytherin
"Fools who wear their hearts proudly on their sleeves, who cannot control their emotions, who wallow in sad memories and allow themselves to be provoked this easily — weak people, in other words — they stand no chance against (the Dark Lord's) powers!" With these words, Severus Snape explained the benefit of Occlumency and revealed a key to his enigmatic character.
As a double agent, Snape well used his ability to keep his true feelings to himself, managing to fool the most powerful wizard in the world, Albus Dumbledore. Dumbledore provided him cloak of legitimacy which all of (Dumbledore's) followers and supporters had to accept. Snape was potions master and a member of the Order of the Phoenix while covertly remaining loyal to the values of Voldemort and the Death Eaters.
To Death Eaters, it is results that count. Power is King. Some Death Eaters, such as Barty Crouch and Bellatrix LeStrange, had the mistaken idea that personal loyalty to Voldemort was the one thing he most valued. But power and ego are opposed to loyalty. Those who clung to loyalty over power romanticized Voldemort and showed they still valued love (if only for Voldemort) more than power. Snape explained how wrong Bellatrix was when he explained how his opportunism trumped her loyal sacrifice:
"You ask why I did not attempt to find him when he vanished. For the same reason that Avery, Yaxley, the Carrows, Greyback, Lucius" — he inclined his head slightly to Narcissa — "and many others did not attempt to find him. I believed him finished. I am not proud of it, I was wrong, but there it is.... If he had not forgiven we who lost faith at the time, he would have very few followers left.""He'd have me!" said Bellatrix passionately. "I, who spent many years in Azkaban for him!"
"Yes, indeed, most admirable," said Snape in a bored voice. "Of course, you weren't a lot of use to him in prison, but the gesture was undoubtedely fine —"
"Gesture!" she shrieked; in her fury she looked slightlly mad. "While I endured the dementors, you remained at Hogwarts, comfortably playing Dumbledore's pet!"
"Not quite," said Snape calmly. "He wouldn't give me the Defense Against the Dark Arts job, you know. Seemed to think it might, ah, bring about a relapse ... tempt me into my old ways."
"This was your sacrifice for the Dark Lord, not to teach your favorite subject?" she jeered. "Why did you stay there all that time, Snape? Still spying on Dumbledore for a master you believed dead?"
"Hardly," said Snape, "although the Dark Lord is pleased that I never deserted my post: I had sixteen years of information on Dumbledore to give him when he returned, a rather more useful welcome-back present than endless reminiscences of how unpleasant Azkaban is...."
"But you stayed —"
"Yes, Bellatrix, I stayed," said Snape, betraying a hint of impatience for the first time. "I had a comfortable job that I preferred to a stint in Azkaban. They were rounding up the Death Eaters, you know. Dumbledore's protection kept me out of jail; it was most convenient and I used it. I repeat: The Dark Lord does not complain that I stayed, so I do not see why you do."
But, to Bellatrix's shock, Snape made the Unbreakable Vow with Narcissa. When it came time to show his true colors, Snape was all action. He showed a complete absence of ambivilence.
Snape said nothing, but walked forward and pushed Malfoy roughly out of the way. The three Death Eaters fell back without a word. Even the werewolf seemed cowed.Snape gazed for a moment at Dumbledore, and there was revulsion and hatred etched in the harsh lines of his face.
"Severus...please..."
Snape raised his wand and pointed it directly at Dumbledore.
"Avada Kedavra!"
A jet of green light shot from the end of Snape's wand and hit Dumbledore squarely in the chest.
The chapter in which Snape kills Dumbledore is called The Lightning-Struck Tower, which, in the tarot deck, corresponds to the path on the Tree of Life known as Peh. It is a horizontal path connecting the pillar of Mercy and the pillar of Severity. Here we have Mercy (Dumbledore) and Severity (Snape) interacting in a bolt of lightning. Two value systems collided and severity has won (for now).