In morals, as in logic, you can't explain something by appealing to something else unless the chain terminates in an axiom.

The question "why is it bad to rape and murder?" can be rephrased as, "how can we determine if a thing is bad, in the case of rape and murder?"

The answer "rape and murder are bad by definition" may be unsatisfying, but at least it's a workable way: everything on the list is bad, everything else is not. But the answer "because they make others sad" assumes you can determine making others sad is bad. You substitute one question for another, and unless we keep asking why, we won't have answered the original question.