The unforgivable curses. Avada Kedavra. As with so many things in life, you can talk about them as much as you want, it's never the same as experiencing it yourself.
The scholars explain that these curses require not only might and skill, but also willpower, the pure intent to kill. And yet none of those scholars are able to cast these curses themselves.
Adepts of the Dark Arts, who merrily cast these curses on a daily basis, simply call the source of their power hate, but even this term is too inaccurate and they themselves do not understand what they are invoking.
Severus Snape was probably the only wizard who ever truly understood the nature of the Death Curse, and it was to remain his secret to the end. Doesn't every wizard, witch, perhaps every human or even every thinking and feeling being have their secrets?
The Dark Lord undoubtedly had hundreds of them, and perhaps it was his secrets that ultimately led to his downfall. Albus Dumbledore, his eternal adversary, kept his cards close to his chest as well, perhaps being a bit more trusting with those close to him.
Snape knew there were too many questions he would never get answers to after this, but he had to do what he had to do.
As the green light of the Death Curse penetrated Dumbledore's chest and the schoolmaster died, he revealed to Snape his final secret, perhaps the most important and cruel of them all.