One of my many favorite things about the Harry Potter series is that JK mostly refrains from making her characters either completely evil or completely good. Her heroes – Harry, Dumbledore, Sirius Sirius – have flaws. Her villians are either humanized – Voldemort, Malfoy – or non-Death Eaters – Umbridge. Slughorn continues this grand tradition by giving us a character who is ostensibly one of the good guys, while being truly impossible to really like.
Slughorn is, at his core, just a really selfish person who doesn’t really care about anyone else in the world as long as he’s comfortable. Harry thinks of him as a giant spider, collecting his “prizes” (former students who went on to do great things) in his web and twitching the strings to ensure his comfort, and I think that’s pretty accurate. He obviously isn’t evil – after all, he’s horrified at the idea that his advice could have led to Voldemort’s seeming immortality – but he just doesn’t care much about the big picture. Slughorn has no interest in serving a greater good or offering his talents to the world – as long as he has his crystallized pineapple and oak-matured mead and a comfortable, safe place to stay, he’s pleased with himself.
It’s another friendly reminder from JK that the world is not split into Good People and Death Eaters (simple as that may be), and that just because someone isn’t a Death Eater doesn’t mean they’re implicitly worthy of your trust.