I'm going to tell my liberal roommate that a youtuber he just discovered is a self-proclaimed white supremacist
How do you know this YouTuber is a white supremacist?
What else would you call a white person who dismisses all of the progress we've made towards making things fair and vocally that whites are the best?
That's a very aggressive stance and may not be true to how they act in real life. There's a good chance they keep their YouTube life and regular life separate to keep up a persona. Either way, you shouldn't try to ruin your friend's enjoyment by telling them things like this.
That's not my problem, they deserve to be made fun of.
Everyone is entitled to their political opinion, even if it's a bit brash, and you should try to respect that even if you don't agree with them.