Yes, I think it's that exactly, especially if they're not Snape apologists. The thing is, though, the book does tell about a lot of great things he's done: i.e. befriend a werewolf, display the extreme loyalty/bravery of doing illegal magic for him, take in his black sheep friend from a scary family, financial support his friends, give Dumbledore a fucking amazing family heirloom, fight Voldemort, etc. I mean, come on!