I don't think it's totally honest to cast him out as an overt, militant racist as it is to put the focus on why people of this generation and this era continue to find this word okay to use in casual conversation. We can probably easiest presume Pewdiepie lives in a social circle where using this word casually is simply okay amongst friends. This is not to excuse his behavior, just to explain how it happened. My presumption is that like many before him, his guard was down and he let it out without thinking because this is a word that is not taboo to use amongst his friends. It is used facetiously, almost as an additional curse word, but one that is a little more "effective" just because of the reaction it gets in non-satirical contexts.
Is Pewdiepie racially sensitive? Clearly he is not. Is he overtly racist--someone that hopes and wishes for black people to be harmed, socially or physically? I don't think so and I don't think he's trying to hurt a black person by using the word. My guess is that, like with his nazi-infused content of the past, he simply uses these words in the same vein as South Park has with slurs and pejoratives--but without understanding the purpose those words serve there.
In other words, Pewdiepie is just not a good role model. I think of him, his approach, and content as tasteless and childish with no redeeming entertainment value of any kind. But, I also don't think of him as a racist with some sort of ulterior motive. Much more likely is the fact he's simply immature.