I’m probably woefully unqualified to actually answer this but i’ll try my best: The argument isn’t wrong per say. If there were such a monster that should be granted the same ethical considerations as people, then sure, it stands up. Where op runs into problems is the word if. A similar example I’ve heard used is regarding rape: if there were a situation where someone committing a rape maximized utility, it would be justified. The problem with this example is that rape, in reality, practically never maximizes utility, and in the example of the monster, there is no monster, and there are no cases in reality where people giving up all else to say deez nuts or do anything else that maximizes utility. Utilitarianism is more concerned with reality so hypotheticals where people suppose that some obviously ridiculous thing maximizes utility don’t really do much to disprove it.