Most white supremacists didn't start off with the intentional choice to be white supremacists. They just grew up as the default, assuming they were the centre of everything. This is a view which was reinforced by every message being addressed to them (not just in their language, but also in their dialect and even accent), celebrating them and pandering to them.
When their position at the centre of everything was first challenged is when whiteness suddenly mattered to them: different = a threat.
This phenomenon tracks with nearly every example of majority privilege. (It's not just a Western, or Northern, thing.)
But this gets really strange when colonisation means dominance in wealth and power, but not majority. I grew up in a country in southern Africa that was a former British colony. Colonisation's effect on the country was deep, broad and unpredictable. Internalised prejudice infiltrated the whole culture.