Attention! DH-spoiler (kind of).

It was a bright morning in the first days of May. Albus Dumbledore was sitting behind his desk in his office in Hogwarts School of Witchcraft and Wizardry, and stared through the window. Open letters were lying on the desk in front of him, and the words on the parchment were still echoing in his head.
Hundreds of victims... We concern it as your duty, the excellent wizard you are... Don´t you feel guilty for the people... they suffer for your hesitation... From now on, the  ministry will officially blame you...
Albus snorted softly. Do they really think they can blackmail me?, he thought and nearly smiled. He was happy in Hogwarts – happier than he had expected, and he was still surprised how fulfilling teaching actually was. Perhaps this is my way to apologise, he thought.  To apologise what I once thought and wanted...
But on the other hand, there was only one way to expiate it...
He looked down to the letters and sighed. It were not the words of ministry officials and “important” people that lay on him like a giant burden, it were the words of a normal woman. On a battered parchment, written in broken English, but with pain in every line.

Dear Mister,
I hear you are a very strong wizard so please please help me. My man and older son has Grindelwald already and I want not to loose my little boy. He is all I have still. All here have so much fear and all womans cry nights. Please please my mister if you can rescue us do it.
  
There was no signature, but lots of tears had left stains on the letter. And Albus Dumbledore felt that this woman was right. He had been waiting – hiding, said a little voice in his head – for too long. His personal feelings did not give him the right to let so many innocents suffer; he had been arrogant too act like this.
Not arrogant, he replied to himself. Fearful. I cannot to this, I cannot fight him! I...
Suddenly, there it was, back in front of his eyes, the image that tortured him. Gellert, smiling in the sun of a long-gone summer, sunlight in his hair and spirit in his eyes... It felt like a knife in his heart.
God, why do I have to suffer like that,  Albus thought,  burying his face in his hands.
“I cannot do this, I cannot stand this, it's too much for me! I am the wrong man, look for someone else, leave my alone!” Now he was already talking with himself. What a wreck he became by this...
And immediately, he answered himself. “But there is no one else, that's the problem! They have searched all over the world for years, and there is nobody else who could... could kill...”
His voice broke in the last word. All he wanted was to run away, to flee the pain that he could not stand anymore, to flee the decision they wanted him to make. But in the same moment, he knew that he could not run away; he would never be able to live with the thought that he was a coward.
“Oh, Gellert,” he whispered. “What have you become?”
The memory seemed to smile. Albus' eyes found the mirror, and the look of his own face wiped the image away from his mind. Gellert had not become what he was now. He had always been the man he was, and he, Albus, had been too blind to see it.
“I did not want to see it,” he whispered. “The brilliance of his mind... his beauty and spirit...”
Tears were burning in his eyes.  “God, what have I done to please him, to win his smile, his admiration... his love...?”   It was the first time he said it aloud. But still he did not manage to even whisper the answer. Everything. I did everything. I could even have killed my sister... And she is dead. As dead as if I had killed her with my own hands. Who cares whose curse is was... Abe's, Gellerts, mine? She is dead because I... because I just followed my feelings, blindly, without thinking about the rest of the world!  
He looked up again, staring into his own eyes in the mirror, staring at his broken nose, the permanent reminder of his failure.
All the people there... They were suffering, dying, just like Ariana... because he was not strong enough to end the terror. Once again, he had the choice to do the right thing.  To purge what happened in this summer of 1899...
Perhaps now he had the chance to end his tortures. Perhaps now it was time to end everything,  to free himself from the doubts, the guilt, the pain. Now... or never. It did not matter. It had to be done, and he was the only one who could do it. If it freed him from his demons, the better. If it made them stronger... Well, his life could hardly get worse. And if so... There were poisons, to sleep long and deep... deep enough to never wake up again.
Albus Dumbledore smiled sadly and stood up. It was time to act. To act fast, before courage left him. He threw over his travelling cloak and left the office.