Let me be clear that I’m open to the idea that I’m seeing it wrong and Kevin is a Changed main character. For now I’ll argue for Steadfast because I still think that position deserves some advocacy …
I didn’t see him as hating his family from the beginning. His very first line is “Mom, Uncle Frank won’t let me watch the movie, but the big kids can. Why can’t I? … It’s not even rated R.” He goes on to complain about “nobody’ll let me do anything”. Just after that is the suitcase bit, which I think is crucial. Kevin admits he doesn’t know how to pack a suitcase because he’s never done it before, and politely tries to get advice on how to actually do it. But no one will help him, they just repeat that he’s helpless and incompetent.
Even when his mom singles him out and punishes him, he doesn’t start out saying he hates them; it’s the other way around: “Everyone in this family hates me”. Only at her suggestion does he wish for them to disappear:
KATE: Then maybe you should ask Santa for a new family.
KEVIN: I don’t want a new family. I don’t want any family. Families suck!
I thought his personal issues were well established prior to that. Not about hating his family but about being stuck being a little kid, who’s a lot more capable than everyone gives him credit for. It’s that “I’m more capable and grown-up than you think” perspective that he’s steadfast on.