Morality is Subjective

 
True morality (as opposed to authority-coerced "morality"), like economic values or taste, must be subjective (or must have subjective premises).  Because if it is not subjective, one cannot truly believe it. Instead one's expressed morality is no more than lip service. One believes assertions, when one either senses it directly, or has a model of how the world works in which that assertion is a part. (A trivial model would be a mental universe in which the messenger of the assertion would not have a motive to lie about the assertion.) Moral premises, on the other hand, are not based on sensory input, but are  extrapolated from innate disinclinations, culture, etc.

Stalin may have done what he did because he was sadistic, nihilistic, fatalistic, idealistic, power hungry, or many of other reasons. That someone finds what he did wrong, is a conclusion based on the individual's indisputable ethical premises, which may or may not involve an assessment of the mental state of Stalin when he ordered millions to their death.  To be able to convince you that something is wrong, I must know your premises (such as, "Humans have an ethical obligation to refrain from behaviors which inflict suffering upon other sentient beings." or "Guide your life by compassion."  (buddhism) or  "If it feels good, do it."(hedonistic) or "Treat others as you would have them treat you."(Christian) or "Maximize the social good." (socialistic, Dr. Spock) "Don't coerce others." (libertarian)), but I cannot argue with your premises because they are subjective. To put forth an argument that resonates with me; however, you would need to know or presume my premises.

Just as there is no objective economic value of a particular thing (only objective selling prices), there is no objective morality (only objective expressed laws and customs (analogous to selling prices)).  If you have no moral first principals (premises) or at least an innate set of vague disinclinations from which to posit moral premises, then any moral argument is pointless.