Both say: “Murder, boo!” thereby expressing their disapproval for murder. So, both disapprove of murder.
However, the subjectivist takes his own personal disapproval to be grounds for the truth of the proposition “Murder is wrong.”
The emotivist demurs. For him, “Murder, boo!” and the disapproval are phenomena of one’s vomiting out his feelings on another without any epistemological import. Nothing is being affirmed as true or false.
As a result, it’s not that the emotivist’s judgments are false; rather they are entirely meaningless and even costly for the judging individual: why should he go to the trouble of “expressing” his feelings to another? And why should his interlocutor bother to listen to such melodrama?
It follows that making emotivist “moral judgments” is irrational, and with that the entire discipline of ethics vanishes away. And that’s a potent reductio of emotivism.