[gregoreous]

"He essentially defines an act as "good" if it's a net increase in human happiness and well-being."

John Rawls, one of the foremost liberal philosophers of our time spends the bulk of his most famous book arguing agains that very idea. My take away is that from a secular perspective it is impossible to argue from first principles what is good and bad. It's also obviously not a very strong argument. If there was a tiny country that did dispicable things to it's citizens but it had nuclear weapons, by this argument, the 'good' thing would be to allow those citizens to continue to suffer rather then have the world intervene and potentially be obliterated in a nuclear war. However, the citizen's of that country would still be correct to say that the rest of the world was unjust.