In Snape’s defence ...

Given the circumstances, what other answer could he give? He knew that Bellatrix would “carry my words back to the others who whisper behind my back and carry false tales of my treachery to the Dark Lord”. Once he made the Unbreakable Vow to undertake that hateful task, Snape had committed himself to carry it out or die and, during his argument with Dumbledore, Dumbledore must have been insisting Snape go through with it. The vow made it inevitable that one of them must die and it is likely that Dumbledore decided Snape’s survival was more important than his own in the ongoing fight against He Who Must Not Be Named.

We know that Dumbledore exacted a high degree of loyalty from those working with him against He Who Must Not Be Named - loyalty to the cause rather than to him personally.

The tasks he set the Order were not easy - he sent Hagrid as an emissary to the giants, Lupin to live among the werewolves, and Snape to spy on He Who Must Not Be Named. When taking Harry in search of the Horcrux he demanded, “If I tell you to leave me, and save yourself, you will do as I tell you?” Dedication beyond personal loyalty was required. Dumbledore did not see the fight against He Who Must Not Be Named as a one-person crusade.

After Harry prevented the Dark Lord from acquiring the Philosopher’s Stone, Dumbledore told him, “while you may only have delayed his return to power, it will merely take someone else who is prepared to fight what seems a losing battle next time”.

In killing Dumbledore, Snape publicly declared his allegiance to the Dark Lord and left himself well placed to assist with his defeat from within the Death Eaters, having once and for all quashed doubts on both sides. Narcissa stated, “[The Dark Lord] would reward you beyond all of us”. As Dumbledore’s man, Snape would have been horrified at the prospect and preferred to die himself rather than murder Dumbledore.

Consider for a moment the quandary of a brave man, prepared to die in the fight against He Who Must Not Be Named. Dumbledore asked him to go a terrible step further and to kill his own leader in the pursuit of the cause. The decision would have been an agonising one for Snape - to die or to kill his mentor, the only one who had ever completely believed in him.

This explains why it was the word “coward” flung at him by Harry Potter that visibly tortured him during his flight from the scene of Dumbledore’s death - a scene where he had committed an act of loyalty which would bring him no accolades but instead brand him with the worst of the Death Eaters and mark him as the Ministry’s next Most Wanted man after He Who Must Not Be Named himself.

From Dumbledore’s point of view, the choice may have been obvious. The Unbreakable Vow meant either he or Snape must die. He was increasingly aware of his own age, “an old man’s mistake”, “I am undoubtedly slower than I was”, and “weaker resistance, slower reflexes ... Old age, in short”.

Snape, on the other hand, is relatively young and at the height of his powers; is a powerful wizard in his own right, an accomplished potion maker, spell maker and Occlumens. Moreover, he has, as much as anyone has, the Dark Lord’s confidence. Even Narcissa was adamant, “He trusts you so, Severus”. Dumbledore calmly accepted serious injury in his work for the Order, “a withered hand does not seem an unreasonable exchange for a seventh of Voldemort’s soul”. He also had no fear of death, “it is the unknown we fear when we look upon death and darkness, nothing more” and “it really is like going to bed after a very, very long day. After all, to the well-organised mind, death is but the next great adventure.”

The fact that Dumbledore was prepared to order Harry to leave him and save himself when they went in search of the Horcrux proves his acceptance that his own death may be necessary in the fight against He Who Must Not Be Named and that he accepted that probability quite objectively.

This is not to suggest that Dumbledore thought lightly of the task - “killing is not nearly as easy as the innocent believe” - but he may have accepted that his time was coming and that others would need to carry on the fight against evil without him.

We know that Dumbledore had a good reason for trusting Snape. “He always hinted that he had an iron-clad reason for trusting Snape.” Dumbledore himself said, “I trust Severus Snape completely” and we know that he trusted few people so completely. Even Professor McGonagall, his trusted Vice Principal and fellow member of the Order, did not know where he went on his excursions from Hogwarts.