He erases all prejudice about Slytherins
Students and wizards who were sorted into the Slytherin House at Hogwarts are often targets of bad stigma. They are perceived as untrustworthy and shady at best, while many are automatically presumed evil and unsavory.
While Snape certainly has contributed to this as well by sacrificing his whole life to undermine Voldemort, he has been on the dark side of the spectrum himself. Slughorn is a powerful and good wizard, who never used unforgivable curses.
He has proven all stigma about Slytherins untrue. He has put a final stop to all this during the Battle of Hogwarts, where he sided against Voldemort and dark wizards, many of them his former pupils, who have attempted to recruit him.
One of very few wizards Voldemort admired
Voldemort did not just pretend to admire Slughorn to extract information about the Horcrux spell. He truly did admire his magical power and skill as a potion master. Some readers perhaps do not realize the magnitude of this.
Voldemort rarely acknowledged anyone’s skill as sufficient, let alone admire it. Admiration can be interpreted as a form of submission, and Voldemort would never let himself be mistaken for such. Against all his ambition and pride, Slughorn earned his admiration. That speaks volumes.
Slughorn fought Voldemort and lived
Slughorn was one of the four wizards who fought Voldemort wand-against-wand during the Battle of Hogwarts. This happened at the same time Molly Weasley dueled Bellatrix Lestrange. He was not alone tough.
It was a three against one duel, with Voldemort on one side and Horrace Slughorn, Minerva McGonagall, and Kingsley Shackelbolt on the other. Even though they outnumbered him, surviving such a fight is proof enough of extraordinary skill.
Slughorn family is in the “Sacred 28”
J.K. Rowling revealed this hidden fact herself. She said that the Slughorn family is one of the twenty-eight pure-blood families that were selected for a book titled “Sacred Twenty-Eight”. Although the book’s author is unknown, it is widely believed that it was Cantankerus Nott.
Horace’s parents and ancestors were never militant in their pure-blood beliefs. They were no way near to the idea as the dark wizards who followed the teachings of Salazar Slytherin on the matter. But, they did have a kind of innate sense of family superiority.
The meaning of the Slughorn surname
This is another fact revealed by the writer herself, although fans did notice many people or spell names to have meaning in Latin. Horace was the name of one of the greatest poets from the Roman Empire.
The poet’s full name was Quintus Horatius Flaccus, and J.K. Rowling was inspired by his Christian name. Slughorn’s name is a combination of the words “Sluagh-ghairm” meaning war cry in Gaelic, which later morphed to Slughorn when a battle trumpet was used for the same purpose.
This original Gaelic term suggests that there is a hidden ferocity in a seemingly harmless appearance. Also, a corrupted version of the word suggests a land slug, which somewhat fits a sedentary man. Horn is also used for announcing famous names and Slughorn did gather people whose names echoed in the Wizarding World.