nigger is only insulting because it's a derogatory word for black people. It's a dehumanizing term to indicate someone is less than human
if you use it as an insult to somebody regardless of their race, you're basically normalizing "you are less than an actual human being, and I'm using black people as the bar"
Like, how the fuck does this have to be explained?
People like Pewdiepie and bootsthecat use the word as an insult because being called a nigger is insulting, because then you're considered black, and being black is considered a bad thing to these people.
It's that simple. If you use the word as an insult or when you're angry, you're a racist.
Because if you're not a racist, then why would you choose that word out of all the words available to you?