Snape had never - rarely - taken pleasure in using the Unforgivable Curses, but it had never cost him effort either. Unlike other Death Eaters, he had used the Cruciatus curse only when it seemed essential to his mission, employed it selectively. Like a Muggle surgeon using a scalpel to expose flesh and organs without feeling satisfaction from the cut, he had used the Cruciatus simply to get the job done. Still, he could not have said how many times.
But Snape knew exactly how many times he had used the Death Curse, Avada Kedavra. At least against humans.
It had never been difficult for him, he had needed no guidance, no practice. When it was time to kill, the green spark of deadly magic came out of him as naturally and effortlessly as the melody from a songbird.
Only Snape knew that scholars and Death Eaters alike were right, and yet could never fully comprehend the nature of the Death Curse.
He carried hatred inside him, an inexhaustible source. More fiery and destructive than the hatred of other Death Eaters, who, driven by cruelty and arrogance, took pleasure in tormenting their prey. More intense and tireless than the hatred of their victims, the righteous anger that drove Aurors and desperate heroes to oppose them.
Fate, in its cruel irony, had forced him to execute Albus Dumbledore. The only man who had ever believed in him, who had rehabilitated him. Who had given him a chance to restore purpose to his life that had been thought forfeit.
Snape stared at Dumbledore for a moment, and disgust and hatred marked the hard features of his face.
"Severus... Please..." said Dumbledore.
Yes, it takes genuine hatred to cast Avada Kedavra. The will to kill is a basic requirement, and it must come from deep within the wizard.
But it was a task, and Snape would accomplish it as he had accomplished every task, stoically and unflinchingly.
For this was Severus Snape's secret: Every single day since then he regretted not having taken that poison.
"Avada Kedavra!"