Assuming he really is a ninja, he would go till the end with the last opponent since he'd have nothing to lose anymore. But then again, he didn't go far enough with Gon, so he's not much of a ninja. Gon simply won Hanzo in the battle of wills, convincing him that no matter what he did, Gon wouldn't budge. If that had proven right, the fight would have been the first and last because the one of them dying of thirst (or some other reason) after an indeterminable time, possibly days, would have made the other one the loser of the whole thing. However, Gon is still a kid so some heavier torture might have changed his opinion. Hanzo simply wasn't enough of a ninja to seek out that limit, especially in front of an audience.