In fairness, Peterson's post about this focuses more on Napoleon than on Kamehameha. Conquest is indeed a bizarre form of greatness. It usually amounts to greatness in crime.
There is some reason to think that Kamehameha may have turned over a new leaf once he had established his rule, so that he perhaps fits the mold of Chandragupta Maurya, whom Peterson (along with pretty much everyone) praises as one of history's few truly great rulers. I found a
story that is supposed to have explained the origin of Kamehameha's law criminalizing violence against women, children, and the elderly. As supreme king Kamehameha tracked down two fishermen who had once knocked him out with a paddle while he was raiding them; he supposedly apologized for the raid and rewarded them. I don't know how much propaganda may have been involved in this story, but Kamehameha lived in the late 18th century and had personal contact with Americans, so his history is not just all ancient legend. And furthermore, even just propagating such a story as propaganda would show that Kamehameha as king wanted to be thought of as great for more than just successful violence.
And even violent conquest can have a greatness that modern people may find hard to imagine. The lands that a great conquerer forcefully unified were often not peaceful countries but savage anarchies in which most people suffered war all the time, with peace coming only after the conquest. People really did welcome that kind of conquest. Successful conquerers would definitely want to spin it that way, but it may really not have needed so much spinning. My admittedly thin and amateur understanding of history and anthropology makes me think that small-scale societies have tended to be horribly violent.
Modern people may feel guilty about condemning those more "primitive" societies, and want to disavow judging them by "our" standards. People may want to justify those societies' customs, whatever they were, as being somehow just as legitimate as ours. I think that's racist, because it amounts to saying that massacre and slavery and human sacrifice were just fine for those savages, who didn't deserve human rights as we do. Kamehameha may not have been the ideal ruler, but he seems to have improved things a lot from how they were when he started, and that really is great.
I was a teenager before it was cool.