You're wondering why? How? What?
So, I could give you my very lengthy explanation (in which case I would need to have the book of front of me, which I don't, because that, like so many other things, is currently residing in Chicago.)
Or I could just tell you that there is precedent!
In fact, eerily similar precedent.
(By the way, the next person who tells me that J.K. Rowling is so original will be subject to my withering look of disdain.)
Here's the precedent, folks, and now we can all agree that Severus Snape is a good man.
From A Clash of Kings by George R. R. Martin (quite possibly the most fantastic epic fantasy writer ever, definitely the most original, least formulaic, again, fantastic):
- "We may escape them yet," the ranger said, "Or not."
"I'm not afraid to die." It was only half a lie.
"It may not be so easy as that, Jon."
He did not understand. "What do you mean?"
"If we are taken, you must yield."
"Yield?" He blinked in disbelief. The wildlings did not make captives of the men they called the crows. They killed them, except for. . . "They only spare oathbreakers. Those who join them, like Mance Rayder."
"No." He shook his head. "Never. I won't."
"You will. I command it of you."
"Command it? But..."
"Our honor means no more than our lives, so long as the realm is safe. Are you a man of the Night's Watch?"
"There is no but, Jon Snow. You are, or you are not."
Jon sat up straight. "I am."
"Then hear me. If we are taken, you will go over to them, as the wildling girl you captured once urged you. They may demand that you cut your cloak to ribbons, that you swear them an oath on your father's grave, that you curse your brothers and your Lord Commander. You must not balk, whatever is asked of you. Do as they bid you...but in your heart, remember who and what you are. Ride with them, eat with them, fight with them, for as long as it takes. And watch."
"For what?" Jon asked.
"Would that I knew," said Quhorin. "Your wolf saw their diggings int he valley of the Milkwater. What did they seek, in such a bleak and distant place? Did they find it? That is what you must learn, before you return to Lord Mormont and your brothers. That is the duty I lay on you, Jon Snow."
"I'll do as you say," Jon said reluctantly, "but...you will tell them, won't you? The Old Bear, at least? You'll tell him that I never broke my oath."
Qhorin Halfhand gazed at him across the fire, his eyes lost in pools of shadow. "When I see him next, I swear it." He gestured at the fire. "More wood. I want it bright and hot."
"No!" The word burst from Jon's lips before the bonemen could loose. He took two quick steps forward. "We yield!"
"They warned me bastard blood was craven," he heard Qhorin Halfhand say coldly behind him. "I see it is so. Run to your new masters, coward."
Face reddening, Jon descended the slope to where Rattleshirt sat his horse. The wildling stared at him through the eyeholes of his helm and said, "The free folk have no need of cravens."
"He is no craven." One of the archers pulled off her sewn sheepskin helm and shook out a head of shaggy red hair. "This is the Bastard o' Winterfell, who spared me. Let him live."
Jon met Ygritte's eyes, and had no words.
"Let him die," insisted the Lord of Bones. "The black crow is a tricksy bird. I trust him not."
The big spearwife narrowed her eyes and said, "If the crow would join the free folk, let him show us his prowess and prove the truth of him."
"I'll do whatever you ask." The words came hard, but Jon said them.
Rattleshirt's bone armor clattered loudly as he laughed. "Then kill the Halfhand, bastard."
"As if he could," said Qhorin. "Turn, Snow, and die."
And then Qhorin's sword was coming at him and somehow Longclaw leapt upward to block. The force of impact almost knocked the bastard blade from Jon's hand, and sent him staggering backward. You must not balk, whatever is asked of you. He shifted to a two-hand grip, quick enough to deliver a stroke of his own, but the big ranger brushed it aside with contemptous ease. Back and forth they went, black cloaks swirling, the youth's quickness against the savage strength of Qhorin's left-hand cuts. The Halfhand's longsword seemed to be everywhere at once, raining down from one side and then the other, driving him where he would, keeping him off balance. Already he could feel his arms growing numb.
Even when Ghost's teeth closed savagely around the ranger's calf, somehow Qhorin kept his feet. But in that instant, as he twisted, the opening was there. Jon planted and pivoted. The ranger was leaning away, and for an instant it seemed that Jon's slash had not touched him. Then, a string of red tears appeared across the big man's throat, bright as a ruby necklace, and the blood gushed out of him, and Qhorin Halfhand fell.
Ghost's muzzle was dripping red, but only the point of the bastard blade was stained, the last half inch. Jon pulled the direwolf away and knelt with one arm around him. The light was already fading in Qhorin's eyes. "...sharp" he said, lifting his maimed fingers. Then his hand fell, and he was gone.
He knew, he thought numbly. He knew what they would ask of me. He thought of Samwell Tarly then, of Grenn and Dolorous Edd, of Pyp and Toad back at Castle Black. Had he lost them all, as he had lost Bran and Rickon and Rob? Who was he now? What was he?
And reiterated again in A Storm of Swords:
- "They're dogs and he's a wolf," said Jon. "They know he's not their kind." No more than I am yours. But he had his duty to be mindful of, the task Qhorin Halfhand had laid upon him as they shared that final fire- to play the part of turncloak, and find whatever it was that the wildlings had been seeking in the bleak cold wilderness of the Frostfangs.
So, when it comes to Snape, Snape is Jon Snow, of course. Dumbledore is Qhorin Halfhand. In the same way that Qhorin commands Jon to do whatever is asked of him, knowing full well that he is going to die, Dumbledore commands Snape (in the woods, when Hagrid overhears) to do whatever is asked of him, even killing him. When Dumbledore begs and says, "Please...please..." he is begging Snape to fulfill this final duty, to kill him. Dumbledore is aware of what this will mean for Snape, how everyone will perceive him, hate him, revile him. He knows why Snape hesitates. But he begs him to kill him...Recall that Dumbledore requests Harry to go find Snape as soon as they return to the castle. Dumbledore knew the time was right, that it was his time to die, to be killed. Notice that Snape has ample opportunity to kill Harry (throughout all the books, when he's teaching him Occlumency, but most especially when he is running with Draco Malfoy and hurling curses over his shoulder at Harry.) Why hurl those curses? Why not kill Harry with one well-placed curse?
Because, of course, Snape is good.
If it should happen that I am wrong, and that J.K. Rowling makes Snape into a bad person, it will definitely be the stupidest, most idiotic, formulaic, move of all of the books, because it would be awesomely predictable, make Dumbledore look like a fool, and suggest that Harry's paranoia was actually good sense. In other words, it would ruin her entire series.
So for the good of her own series, Severus Snape must be a good man.
Of course, knowing JK Rowling, it's entirely possible she opts for ruining her series.
But I hope not. Because God knows it would be far more interesting to see the look on Harry's face when he realizes he's trying to kill the one good man throughout all the books, the man who has to suffer so much and bear so much in order to help save them all, the misjudged, hated and reviled hero...Severus Snape.