The fact that he was never protrayed in a sympathetic light. From the beginning he was torturing orphans, hanging bunnies, stealing, etc. It was as though he was born evil. If he was in a living nightmare of an orphanage (and to be honest, most of them were. Oliver Twist is factual in that regard) then I could understand. What was more, if JKR could have contradicted herself more blatently with the whole 'Choice vs ability' theme, it was here.

As for the second, again, Zabini is not protrayed in anything resembling a sympathetic light, and even Slughorn seems creepy and is just an obstacle for Harry to overcome.