Does this text sound authentic to you?

Statements on this rather low rhetorical level would always omit the relative pronoun: "It's so dumb it makes me barf."

A reasonably educated young person over, say, the age of 15 would also be able to construct a better argument, so unless you're writing this for a character younger than this, I'd tone it down a bit and add little more substance. For some reason, perhaps the apologetic "I don't mean to be repetitive," I'd guess this was spoken by a young woman.

"Barf" is also a rather old slang word. A younger person would use a more recent expression. If the person is male, then this expression might be quite colorful.

"When you kill someone, they..." Although strictly incorrect, the use of the plural "they" with a singular antecedent is so common today in both British and American English, it would look very odd if you used any other expression.

So, except for the relative pronoun, it's an authentic text.

Waaaaaaaay too over the top. Don't use the word "preposterous" (hardly anyone uses that), cut the slang ("barf"), and the "I bet they literally have no brains" seems a bit too much/unprofessional :) What's it for?

It is a little much ... I would just state " I believe the hunting of animals should only be done to feed and clothe people. I think killing animals for sport is disgusting."