What a wonderful double portrait. I enjoy seeing Slughorn portrayed as a pragmatist with a conscience, someone whose self-indulgence is balanced by his insecurity. I love that he's a hedonist and a selfish man and yet capable of decency and regret. The depiction of Cornelius is also very nicely handled, because his abject misery at the beginning makes the return of his sharp, ambitious mind a sign of health, something the reader rejoices to see. I love all the little details of daily care, and the sense that Slughorn's been haunted for years by his own failure, his own betrayal of his students and by extension the world, much as Fudge is responsible for denying reality and helping to bring horror down upon them all. It's appropriate that these two guilty men should find forgiveness in each other.
The writing is both careful and quick, never flagging, grounded in a real world but imbued with so much emotional nuance. It's quite sorrowful, really. And some of the insights are so perfectly phrased. I think I loved these lines above all, because I actually had to stop and read them twice when I encountered them: it was perfectly natural to be a little haunted given that such a large thorn had recently been extracted from his side. The problem was that Tom, like many thorns, tended to leave traces in his victims. This is so much more disquieting in demonstrating the effect Tom Riddle had on those he used and threw away, and how his poison seeped into them and left a taint.