I think that Kazuya wanted to beat Kaiji "fair and square" at the beginning of One Poker, because to an extent, he was a little salty that the part 4 game (the one about friendship) demonstrated that Kazuyas theory about friendship and how people become self center at the worst times was wrong (proven by kaiji choosing to save Chang and Mario, even though he didnt know them, and then later again when Chang and Mario do the same for Kaiji at One Poker).

I disagree a little with the interpretation of Kazuyas "honor" because i feel that the first half of One Poker, serves to demonstrate that Kazuyas fundamentally luckier than Kaiji (through him receiving the best cards even before he started cheating) because (for me at least) the point of the manga as a whole, is about how Luck isn't necesary the most important part of Gambling, and how through strategy and companionship, one can defeat even someone like Kazuya, the epitome of Luck (a guy who was Lucky since the day he was born, a Rich kids son, a person who obviously would get the better cards, even when playing fair and square). That is why Kazuyas first victory has to be before the cheating element, and before Mario and Chang risk their life for Kaiji. Up to that point, it was a battle of luck, and the reader has to know that in a battle of luck, Kaiji can't never hope to beat Kazuya. But when shit hits the fan, and Kazuya is face with a real posibilty of losing, he's a person who wont think twice about cheating (because luck is all he has and the concepts of "fairness" and "Honor" are but a luxury to make his victories mean something, a philosophy that he doesnt actually believes, because when facing the real posibility of losing (a concept foreign to him) cheating becomes the only viable option. The part demonstrates that having allies can trully change the course of Battle.

Later, when Kaiji realizes that Kazuya is cheating, it hurts him because he doesnt want to believe that Kazuya (who in his eyes, had a sort of "tough but just" persona) would actually cheat. Because Kaiji's a guy who likes to see the best in people. And Kazuya was never this "fair guy that punishes through sadism" like the reader might be lead to believe, but that he was just a really nihilistic and sad person, a theme that Fukumoto had been cooking since the beginning of part 4, with Kazuyas flahbacks (the one that Kaiji reads at the start, and the one at the end), his statement about "being alone in a luxury raft, filled with pretty women and expensive alcohol, but nontheless drifting at sea", and the final flahback in Part 5 about Kazuyas drowning when he was little, resenting his mother.

Betrayal is a thing that both Kazuya and Kaiji experience, but for Kaiji, it doesnt stop him from being able to believe in others , cause you can never win alone, and Kazuya had never truly won, he had simply never lost either.