How we treat others is really in the end how we treat ourselves. Our outer conduct always has inner results.
In a great little passage from Shakespeare's play, Hamlet, the Prince addresses Polonius about some theatrical players who are visiting, and we get this exchange.
Hamlet: Good my lord, will you see the players well bestowed? Do you hear, let them be well used, for they are the abstract and brief chronicles of the time; after your death you were better have a bad epitaph than their ill report while you live.
Polonius: My lord, I will use them according to their desert.
Hamlet: God's bodkin, man, much better. Use every man after his desert, and who shall 'scape whipping? Use them after your own honor and dignity. The less they deserve, the more merit is in your bounty. Take them in.
Notice the evolution of the reasoning Hamlet uses with his friend. At first, he asks Polonius to "use" or treat the players well, and appeals to his self-interest in a fairly superficial way, pointing out that these are people whose job is in part to broadcast news and gossip far and wide, and that they'll certainly tell tales of Polonius, depending on how he treats them. If he treats them well, he will be well spoken of everywhere they go, and if the opposite, then his reputation will surely suffer. But Polonius objects, on what looks at first to be moral grounds. He won't treat them well just because he'd benefit from that - he'll hold to higher ground and treat them the way they deserve to be treated.
Hamlet feigns shock at that declaration, and jokingly points out that, on this principle, any of us would be lucky to escape a public whipping. He then suggests that the better course is not to treat others in accordance with their character or merits, but rather in accordance with our own honor and dignity.
The high path of moral action is to act well toward others because of who we are, not just in response to who they are.
Our actions should express our higher nature, and there are four distinct benefits from that.
First, by acting out of honor and dignity and treating others well, we set a high moral tone of kindness, rather than just responding to others in kind. We are moral leaders, rather than just reactive puppets who allow our own conduct to be dictated by others. Second, by acting well, we reinforce our own ideals and higher tendencies. Whenever we act, we never just do, we always become. Third, kindness, generosity, and mercy do in fact, more often than not, generate the good report of others, and this reputation indeed will serve us well in the hearts and minds of other good people. And, fourth, we should be reminded of the words once spoken by Goethe, when he said:
Treat others as if they were what they ought to be, and you help them to become what they're capable of being.
By treating others well, we make gains, however small, in surrounding ourselves with the sort of people who are good company and good partners in making great things happen.
When we do well, things tend to go well in many ways.