Each person is born with a purpose. Whether the person knows his or her purpose doesn't matter. But what does matter is when that purpose is complete, the person will know. The person will know because the driving force that pushed him or her through life will know longer urge the person to keep going. Suddenly, the person wonders what he or she is doing here, why he or she lives. Every now and then a person will find a new purpose. Perhaps this purpose is to help others achieve their purpose, perhaps it is to continue a lost purpose, or perhaps it is to delay the inevitable. Yet no matter how many people think they chose their own purpose, they are wrong. Purposes will choose the people, much the same as wands choose the wizard.
And so, a certain person has completed the purpose that chose him. His purpose was greater than others in some regards, but so much less in other regards. He knew his purpose was complete. He wanted a new purpose; he wanted to delay the inevitable. Yet no purpose could take the place of the purpose of the one he had been born with. No purpose seemed to satisfy him, yet no purpose was simple enough. He was tired; his original purpose wore him out.
The moonlight shone in his room as he looked at the slender piece of wood. His purpose was complete and there could be no other purpose for him. No purpose could fit him. He could fit no purpose. With one last word on the parchment before him, he took the way of so many purposeless people. He could not live without a purpose. He had no driving force to keep him waking each morning and resting each night. His purpose complete, he said two final words.
With those two final words, the purposeless man slumped in his chair. With those two final words, the purposeless man defeated the defeater of the Dark Lord. With those two words, the purposeless man rid the world of a purposeless Harry Potter.