You can't tell people what they ought to do based on a fact of what's good or bad.
There is a whole branch of ethics that deals with complex circumstances where the weighting certain actions that have different moral values applied to them is considered. For example, a man who steals from a street vendor to feed his family-- the loss to the street vendor is less than the good of feeding his family. This is acceptable. To lie to a Nazi official to save the lives of refugees is acceptable. But these circumstances do not make stealing subjectively good or bad. The circumstances with different outcomes to be weighed are weighed objectively. Stealing is wrong, but feeding is family is a greater good. Lying is wrong, but protecting the refugees is a greater good. Lying is not "good," it is that opposing effects must be weighed. Some situations are much more difficult to to assess, less clearcut.
I think you cut off part of the Socratic dialog. Men do evil, thinking it to be good (good for themselves) but do not take into account the harm they do to others. A man may tell himself I need to have more money to feel secure, so I will pay my employees less, and tell himself it is "necessary" to do so, but he ignores the hardship his employees face. A CEO tells himself he must make a certain profit to retain his job, but he does not consider who will be harmed. Corporations resist changing their manufacturing procedures because it will reduce their profits, but neglect to consider the cost to the environment long term, which affects everyone. In all these situations, it is clear that opposing parties see themselves as acting rightly, but an outsider can clearly see that self-interest drives one party, to the detriment of the other. The moral choice, when weighed without self-interest is usually clear. It is not just Jews and Christians who hold to the belief that stealing, lying, and murder are wrong.
When we consider basic human rights, such as the right to life, the right to liberty, the right to self-determination, it is clear that these can come into conflict also. We deprive a man of liberty because he steals, thus he deprives others of their means of livelihood for example, by stealing their car. A woman chooses an abortion because the pregnancy will affect her schooling. But by doing so, she deprives the future child of its life. Which is the greater harm? Which outcome is the greater good?