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.

How could you possibly judge me if you don't know my backstory? It's not even enough to know it, you'd need to have lived it. Besides, I didn't kill him, but you already knew that.