As I understand it, the argument isn't so much "if you use a thing made by a bad person, you are a bad person by association" but rather that using a commercial product made by a bad person, who spends his money on bad causes, is directly helping him spend more money on said bad causes. Since he has never apologized or shown any indication that he has become a better person, not wanting to monetarily support him is a valid reason to not use his product.

It's really hard for the creator of Javascript to make money off of javascript, and it's unlikely he has any financial interest in the Mozilla corporation anymore since they're a nonprofit and thus don't have share holders. However, he directly profits off of Brave.

Because it is cool thing to cancel everything in 2023.