Professor: Potions, former Slytherin Head of House
Slughorn is the newest Potions teacher at Hogwarts (though this is his second time teaching there), and, boy, does he love famous people. We think Dumbledore describes him 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 backseat – more room to spread out, you see. He used to handpick favorites at Hogwarts, sometimes for their ambition or their brains, sometimes for their charm or their talent, and he had an uncanny knack for choosing those who would go on to become outstanding in their various fields. Horace formed a kind of club of his favorites with himself at the center, making introductions, forging useful contacts between members, and always reaping some kind of benefit in return, whether a free box of his favorite crystallized pineapple or the chance to recommend the next junior member of the Goblin Liaison Office." (4.75)
Just when we are about to give a good eyeroll at Slughorn, we come across an interesting moment. With the help of Felix Felicis, Harry is able to secure the memory that Slughorn has been so reluctant to give up. We see a side of Slughorn that we have never seen before – namely we see intense remorse and self-loathing for the fact that many years ago as a teacher he gave Voldemort key information about Horcruxes. We feel sorry for the Professor when he confesses to Harry: "'I am not proud…" he whispered through his fingers. "I am ashamed of what – of what my memory shows….I think I may have done great damage that day….'" (22.212).
Another point in Sluggy's favor is that he really liked and respected Harry's mom. We feel that respect projected onto Harry as well: "'You're a good boy,' said Professor Slughorn, tears trickling down his fat cheeks into his walrus mustache. 'And you've got her eyes….just don't think too badly of me once you've seen it….'" (22.217). No doubt, Horace is a fame-loving Slytherin through-and-through. But we do want to point out that though he possesses powerful connections, he has never joined forces with Voldemort. He has never stepped over to the Dark side. When we first meet Slughorn, we find he has actually been running from Death Eaters.
Slughorn proves that not all Slytherins are evil. He is good at heart and only wants to live comfortably, knowing his students are all out in the world doing famous and important things. At the end of the day, Horace is just a regular guy who cares about what people think of him and who wants to be important and liked.