Revelation and Concealment
Horace Slughorn loved his job. He also loved his hobby, but he had always believed they were essentially the same.
He loved combining ingredients, creating value out of worthless components, revelling in forming something greater than the sum of its parts.
And he also loved unveiling the potential in people, exposing them to the influences and experiences that would make them great, creating opportunities out of raw talent. People were just like potions ingredients, really; useless sitting in a cupboard, but holding endless possibilities when placed in the right environment.
His was a business of revelation, of bringing things to the limelight. So it was surprising (or maybe it was not) that he kept perhaps the most important moment of his life cloaked in secrecy and a desperate, futile attempt to conceal.
He'd never noticed anything unusual about young Tom before; talented of course, but he had never seemed dangerous. In retrospect, it had been bound to happen sooner or later - the risk of associating with people more powerful than yourself was that one day they would realize it and the game of manipulation would be inverted.
Without his even realizing it, one of magic's ugliest secrets was revealed, and with it Horace understood that Tom's interests were not usual or acceptable. He had known almost immediately that it had been a mistake. He had actually been on his way to Headmaster Dippet's office to discuss the boy's worrying tendencies when a fresh wave of horror rolled over him. He felt unable to continue. No, he had had enough strain for one evening. He would do it the next day.
The next day he remembered that Dippet was an idiot and that he would need to find someone else to talk to. The following week he was marking papers. And Dumbledore never seemed to be around and there was so much to do and was it really that important anyway?
Horace knew that none of the above was true. He knew that the true reason was that he did not want his guilt exposed, and that every day of concealment added another layer of guilt, of wrongness in allowing the situation to continue. It was too hard to talk about it or even think about it; the truth became slowly buried under a blizzard of culpability.
Decades passed. Few recognized Hogwarts' golden boy in the new Lord of Chaos. But not Horace. Had he ever forgotten a student? Had he ever failed to exult in the successes that he enabled? Every time those red eyes glowed on the front page, he knew to whom they belonged, and he knew the evil sorcery that had made them that way. And every time his shame welled up in his throat and stifled the long overdue confession. It would take a very special, lucky boy to uncover the truth.
In years to come, well-meaning people would assure Horace that he was not responsible, that he did not bear the guilt of the rows of graves at Hogwarts and in so many other places. But he knew they were wrong. After all, he was the crucible in which Tom's ambition had been formed; he had added its most poisonous element and had allowed it to fester undisturbed for so many years. As a potions master, he knew that no such venomous concoction could be complete without all its ingredients.