Sound's like you're opposed to all animal research, not specifically brain-computer interface research. We also don't ask a monkey's consent before doing any other sort of experiment on it.
IMO Utilitarianism is a particularly dangerous ethical framework when wielded by narcissists who have a difficult time imagining that they might catastrophically fail (looking at you Sam Bankman-Fried), or might not be delivering salvation to the world (looking at you Elon, lol).
If you tell yourself it's for "the good of humanity", or the alternative is destruction / widespread death, you can justify any action.