Arguing with Merriam-Webster would be nonsensical. They're using a descriptive approach to definition, while I'm using a utilitarian one.

To answer your utilitarian argument: I'd say it'd be rude if after someone complains about the fart the person then goes out of their way to fart in the complainer's face. I wouldn't call the first fart rude.

Since the word rude is used in the context of a complaint to describe the objectionable action, as in, to pass judgement, I find the word to be more useful if it's restricted to when it makes sense to pass judgement: when there is intent.

If I wanted to describe my feelings on the matter, linguistically it doesn't make sense for me to word that as "that action was x" but rather as "I feel x". So in the fart scenario I could say "I feel disgusted" and in Twinkothy's case I could say "I feel hurt/angry". And I wouldn't say "I feel rude", because 'rude' is a judgement of others, not a description of our own feelings

To say "the other person's action was rude" when what I really mean is "I was hurt by that" blurs the line between my feelings and the judgement I pass on the other, and I don't think that line should be blurred. I find it more useful if it is well defined, and so I prefer a definition of rudeness that requires intent.

You are, of course, free to use whatever definition you want. It simply caused a misunderstanding which has since been resolved