How do you know promoting the well-being and happiness of others is moral? What if someone disagrees with that?
Well it's a maxim - which basically means it's fucking obvious to a normal person who thinks about it
Of course some people will disagree - because some people are stupid, or insensitive, or screwed up. Fortunately the idea is obvious enough to have made its way into societal norms, cultures, many religions, and legal systems. So it gets reinforced at many levels and people who don't behave in accordance with it tend to get counselled, ostracized, into fights, or punished.
It just "feels right?" May I remind you that it "felt right" to Mao/Pol/Stalin to do what they did.
What's with the "May I remind you..." tone of voice?
Do you think I'm an idiot living under a rock? And that you are somehow superior at remembering stuff?
How is history treating Mao/Pol/Stalin? Many of their ideas were socially defective and unsustainable. They've been corrected and condemned by the very societies they were hurting, and the rest of the world. That's exactly what you'd expect when a maxim such as the global ethic of reciprocity is violated.
And about empathy and fairness:
What if someone only has empathy for their own family?
Or if Hitler only has empathy for Nazi's?
Did you notice how it didn't work out for Hitler?
Do you have a point?
Heck, even notorious serial killers had empathy for their moms.
Well anything biological, such as mirror neurons, will vary in its exprerssion. Like some people have big noses, others small ... a few people are born completely without noses. So it is also with our hard wired empathy. And then overlayed on that is the cumulative psychological effects of life's experiences. Some people get fucked up by nature, some by life, and some by both. When that happens society eventually has to step in. What's your point?