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Severus had always detested violence.
He had seen and experienced enough of it growing up.
Violence was for people without subtlety, without cunning - for people like James Potter and Sirius Black. The strong and violent do not come to a natural end, and Potter and Black had certainly met the exact ends they'd deserved - without achieving a single meaningful thing in their worthless lives.
Nevertheless, Severus hadn't wanted James Potter to die, as he hadn't wanted Black or Lupin to die. If he had been satisfied with something that simple, that crude, he could've done it himself any time he wanted. No, what Severus had wanted was for the whole world to know precisely what kind of people Potter, Lupin, and Black were - and to have them Kissed as retribution. Instead, James Potter had died a hero, a martyr, and his death had taken the truth with him.
And Lily as well.
That was what drove Severus to Dumbledore.
Regret was a slow, caustic poison - festering and corroding everything until nothing but a hollow husk remained. And hollow husks, much like Inferi, were easy to manipulate and control. Severus had known it, but he hadn't had the strength or will to cut himself free.
Not even when Dumbledore demanded the unthinkable of him.
For all his fascination with the Dark Arts, for all his hatred and contempt, Severus hadn't, not once, taken a life. Even in the first war, he had spent most of it in the backlines, harvesting dead bodies for ingredients and brewing potions for the Dark Lord's troops.
So when Dumbledore showed him his hand and told him it was time, he said no at first. He kept saying no, right until Narcissa came knocking on his door in the middle of the night with her sister, desperate and devastated. She'd been too pure, too proud, to kneel even in front of the Dark Lord - and yet she had kneeled in front of Severus and kissed his hand. And he'd been... touched, the remnants of love he hadn't allowed himself to grow or feel stirring.
Severus had always been a fool when it came to women.
And Dumbledore had already asked him, and the Vow had been vague enough that he could've worked around it if he so chose.
But then Draco, that foolish boy who was too brilliant for his own good, had actually succeeded in the impossible. He had brought the Death Eaters to the school. He had rushed up to meet Dumbledore, as if he ever had it in him to kill - and even if he did, he'd only lose his own innocence and stain his soul.
And when Severus ran after him and saw the members of the Order of the Phoenix in the corridors fighting Death Eaters, he realised why Dumbledore hadn't done anything about Draco's antics for the entire year. Why he hadn't been interested in stopping him, or talking to him, or helping him. Why he'd called the Order into the school before he'd left.
Dumbledore had always believed in freedom only as far as it got him what he wanted.
And when Severus ran up the staircase and slammed through the door, he thought of Narcissa's devastating eyes as she begged him to save her son; of Draco chattering his ear off as he helped Severus bottle potions in the slow afternoons; of Dumbledore's cold blue eyes as he said, "You will"; of a lifetime of violence and manipulation, and for a single fleeting instant he felt a potent enough hatred for an Avada Kedavra.
Severus looked Dumbledore in the eyes as the words fell from his lips.
It was the last courtesy he would give him.