Unless you're the Half-Blood Prince, it seems that most of the Hogwarts students we're familiar with find Potions to be one of the trickier subjects. Compared to the stone-cold Professor Snape, the return of Professor Horace Slughorn and his hands-on teaching practices was a delightful breath of fresh air. That being said, he wasn't without his own dark past.
Slughorn loved to encourage and answer any questions that his students had. However, he inadvertently answered one question that ultimately helped an evil wizard follow out a dark and malicious plan, which he regretted for most of his life.
Never Front and Center
How to describe Horace Slughorn? Dumbledore has honestly said it best:
"Horace likes his comfort. He also likes the company of the famous, the successful, and the powerful. He enjoys the feeling that he influences these people. He has never wanted to occupy the throne himself. He prefers the back-seat; there is more room to spread out."
Slughorn was a pure-blood wizard as well as Head of Slytherin House. He served as Potions Master for over fifty years before retiring and going into hiding.
A Slip of the Lip
When Lord Voldemort was a mere young Tom Riddle, Slughorn could feel the brilliance and power from his young wizard pupil. One fateful day, Riddle approached Slughorn, his motives perfectly calculated.
As seen in the video below, Riddle asked questions regarding horcruxes, one of the most intense and painful pieces of magic done by any wizard.
Notice how terrified Slughorn acted when Riddle asked about the possibility of having multiple Horcruxes? As if having one wasn't bad enough.
Slughorn carried this disgrace for many years, which forced him into hiding and purposely damaged his memory so that he would forget his role in future atrocities committed. Who would have known that this little encounter would lead to the rise of the terror that was Lord Voldemort?
Who Can Resist "The Boy Who Lived?"
Although Slughorn was initially hesitant to return to his post as Potions Master, Dumbledore knew he wouldn't be able to resist Harry's charm and celebrity. Using Harry as bait, he was finally able to convince him to return to Hogwarts.
Dumbledore knew the information Slughorn revealed in his encounter with Tom Riddle was vital to the outcome of the war, and knew that with some prompting, his guilt would make him come forward and face his past.
Slughorn's Return to Hogwarts
Once at Hogwarts, Slughorn started up his "Slug Club" again, his collection of favored students, based on their relations to famous or important people, or on a student's unusual talents that will make them important and famous once graduated from Hogwarts.
Harry was surprised to find that Lily Evans was one such witch who took part in the "Slug Club." Every time Slughorn looked into Harry's eyes, he'd see Lily, and feel the immense guilt that he mistakenly taken part in the death of someone he truly cared for.
Extracting Slughorn's Buried Memory
So Slughorn might be guilty of a terrible memory. But his immense pain for aiding a young Tom Riddle and accidentally planting the Horcrux ideas into his head is a memory he purposely tried to erase.
In Half Blood Prince, Harry takes a swig of Felix Felicis and visits Slughorn, with the intent of extracting this exact memory, in which Slughorn tells Riddle about the risk of having multiple Horcruxes.
Although it takes a bit of coaxing from Harry, with regretful tears in his eyes, Slughorn finally relents and gives Harry this memory, finally showing his courage and his will to defeat the monster he had created.
The Fight to Be Rid of Ultimate Evil
Even though he could have stood his ground and remained firm against Dumbledore's insistence on his return, Slughorn was courageous enough to step up and face his past.
He redeemed himself further during the Battle of Hogwarts by staying to fight and recruiting a large number of reinforcements to help fight the Death Eaters that had barreled through to destroy Hogwarts. Slughorn even goes so far as to join in the duel against Voldemort, along with the likes of Professor McGonagall and Kingsley Shacklebolt.
A Slytherin Unlike the Rest
Whereas most Slytherins refer to Muggle-borns with the incredibly offensive and derogatory term "Mudbloods," even as the Head of House, Slughorn doesn't seem to pay any mind as to a family's blood line. Lily Evans and Hermione Granger were both welcome members of the Slug Club, despite being born from Muggle families.
Also, while he does think about himself first an foremost, and doesn't necessarily follow rules, he isn't truly amoral and does not have any ill intentions behind his actions.
As a fellow Slytherin, I feel offended when people call us evil. We may want to always be ahead of the game at whatever cost, but a lot of us are actually really kind and selfless. Slughorn stepped up when his courage was needed and was able to reverse what he had caused in the first place, redeeming himself as he stepped up to the plate.