The emotivist theory has been criticized quite a bit and is no longer accepted by very many philosophers. I suggest we reject it and work with a different definition of "X is good." If you want to read a detailed criticism of the emotivist theory, I suggest you read J. O. Urmson's book The Emotive Theory of Ethics.

The utilitarian definition of "X is good" as "X will maximize the happiness or welfare of sentient beings" also has problems. In his book Principia Ethica the philosopher G. E. Moore pointed out that if we define good in terms of what he called a natural property, we can still meaningfully asked whether having that property is good. For example, even if we define good as tending to maximize happiness, we can still ask whether an act that maximizes happiness is good. But if we can do this, it appears that the term 'good' does not mean the same as "tending to maximize happiness." (It would make no sense at all to ask whether happiness is happiness.) I suggest that we not try to use the utilitarian definition either. This argument is called the "open question" argument. It is quite famous and many philosophers have accepted it.