I think it would depend on what kind of “white supremacist” one is talking about.
150 years ago, probably most of the European world considered non-Europeans to be truly “inferior” in ability. They looked at things in the non-white world, and saw a lot of chaos, and a lack of societal accomplishment.
Now, they could have taken a look at the Japanese in 1880 and realized non-whites were capable of doing the very same things Europeans were capable of doing, but for the most part, the non-white world was primitive. A Japanese could also have taken a look at how “white” people lived in Siberia and concluded that “whites” were inherently inferior, and by and large that’s exactly what they believed. A lot of them still do. There was, however, an interlude during which that conclusion on the part of the Japanese led to immense atrocities. That’s a different thing. That’s hate.
But regardless, the evidence undoubtedly seemed compelling to them. It wasn’t a matter of hatred or ill will, it was just a matter of reaching factually incorrect conclusions.
There have been studies suggesting that whites are the intellectual superiors of blacks and that Asians are the intellectual superiors of both. How much of that is cultural and how much of it is inherent, I don’t think anybody knows. But regardless, that’s not a “hateful” kind of conclusion, it’s just a conclusion drawn from certain studies. it does not imply ill will of any kind.
One might accuse, e.g., Kipling of being a “white supremacist” in that he seemed fairly clearly to think Europeans (particularly Brits) innately superior to “natives” of various sorts. But he also accepted the idea they were souls equal in the eyes of God and entitled to the respect a human owes another human.
I don’t see that as necessarily evil in its time. We presently have better evidence of the capabilities of non-whites and can reasonably be blamed for insisting they are somehow inferior.
There are some who see non-whites as some kind of enemy that must be suppressed or dominated or even eliminated. That’s hate; i.e., a desire to harm another.
It seems to me nobody should truly believe in the inherent superiority of one “race” over another, because it’s contrary to known facts regarding individuals, even if some studies show differences in the aggregate. To me, insisting on it is not necessarily evil so long as one keeps it to oneself and doesn’t act on it.
What’s truly evil is the desire to harm. What’s even more evil is taking action on it.