Before Dying, Dumbledore Left Us With One Big Mystery, And We Think We Solved It
Before his death, Dumbledore suggested that it was Draco who depended on his mercy and not the other way around. What did the Headmaster mean by that?
In The Half-Blood Prince, before he died, Albus Dumbledore had both a casual and increasingly intense conversation with Draco Malfoy, his wanna-be killer. Despite being held at gunpoint, the Headmaster acted like he was in control.
In fact, he informed Draco that it was his, Dumbledore's, mercy that mattered then, not Draco's.
What did he mean by that? Well, there are at least five ways in which the Headmaster showed his mercy to Draco that night at the Astronomy Tower.
Staying put
Dumbledore was the most powerful wizard of his generation at the very least, and his magical abilities were simply unmatched.
The most obvious definition of mercy in this situation would be "not hurting Draco" as it's clear: even wandless and exhausted, Albus Dumbledore could've easily stopped an armed 16-year-old.
Offering protection
Dumbledore's mercy included directly offering protection to Draco and extending it to his entire family. The Headmaster was ready to forget and forgive the sins of the Malfoys to save them from Lord Voldemort's wrath.
That is, considering Lucius Malfoy's multiple crimes and his son's numerous murder attempts on Dumbledore.
Saving Draco's soul
Throughout their entire dialogue, the Headmaster was repeatedly telling Draco that he was not a killer. This didn't mean that Draco was too weak to become one; it meant that Draco didn't have to.
In fact, Dumbledore had saved his student beforehand by ordering Snape to kill him instead, thus sparing the poor boy's soul.
Protecting the Malfoys
Dumbledore didn't just offer to protect Draco's family: by that point, he'd already done it.
Dumbledore knew about Malfoy Jr.'s quest, and he knew that Lord Voldemort wanted Draco to fail and die as punishment for his parents. Instead of stopping Draco, the Headmaster let him succeed, which saved the Malfoy family.
Giving the final lesson
With their last talk, Albus Dumbledore taught his last lesson to his two students, Draco and Harry. The Headmaster knew he had to die, and he'd planned it himself.
Still, in his final minutes, the old wizard revealed the true power of love: he faced Draco's murder attempt with kindness and consideration for the boy.
With his love, Dumbledore stopped a potential murderer, a desperate man on a quest to kill the Headmaster. The Headmaster saved his soul, his family, and the boy himself one last time — and prevented Draco from killing him.
Dumbledore's last lesson underlined once again that love is, indeed, the most powerful kind of magic.