is there a reason behind Zabini's dislike for Harry
“I wouldn’t touch a filthy little blood traitor like her whatever she looked like.” –Blaise Zabini, Half-Blood Prince
That’s it, that’s the whole reason: blood prejudice.
Actually I have a conversation between Blaise and Draco set in the upcoming volume that delves a little deeper into the subject, but if you jump back to the first story in the series they discussed it in brief then, too:
“He acts like a Muggle-born! And, I mean…his mother was a Mudblood,” [Blaise] whispered with a grimace.
Draco’s response was a scathing glance over the top of his book. “No, really?” he said dryly. “I had no idea.”
“Well, I didn’t,” Blaise muttered.
“No?” Draco asked, clearly disinterested in the answer.
“Well I knew he wasn’t a pureblood, obviously,” Blaise replied, “I just didn’t know it was…that bad. I mean…his mother?”
When Blaise went home for Christmas break during his first year, he learned more details about Harry’s origins, specifically in regards to Lily Evans. He immediately begins acting more coldly toward Harry, purposefully distancing himself, whereas in the early chapters of the first volume he had been pretty friendly toward him (as friendly as Blaise is toward anyone, at least).
I wanted to show that blood prejudice takes different forms, and manifests in different levels, within the Wizarding world – so I had Draco be more pragmatic (like his dad’s family was historically) and “willing to overlook” the “unfortunate circumstances” of Harry’s birth because of who Harry is, and the value inherent in his friendship…while Theodore Nott doesn’t give much of a damn about other people at all, but certainly wouldn’t object to a bit of Muggle-baiting or blood-shaming and probably uses words like “Mudblood” as casually as most people use “shoelace"…and Blaise, well, thanks to his ugly comment about Ginny that I quoted above and the snide way he reacts to Hermione’s parents being Muggle dentists later in HBP, he landed the role of “most overtly blood-prejudiced” out of all the Slytherin boys in Harry’s year.
I’m not sure when/if Harry and Blaise will get to have a confrontation on the topic themselves – maybe in fifth year, the timing seems good, but we’ll see how the story develops as I write it – but there’s your answer: blood prejudice.