The discussion is respectful and I pray beneficial. My interlocutor’s words appear bold and italicized.
“Morality is a social contract that used common sense to protect the individual. I don't want to be killed, neither do you. Let's make a rule that says we can't kill each other!”
I do not accept the claim that morality is merely a "social contract." Murder, rape, pedophilia, etc. are morally wrong regardless of what a particular culture/society may determine.
“Do unto others as you would have done to you was not original to Jesus . . ."
Of course not. (I don't know anyone who says otherwise.)
"We just have to do our best and use common sense . . ."
"Best" for what or whom? Who decides what "best" is? By what standard? (Some of the most heinous crimes against humanity were/are done in the name of the “common good.”)
Furthermore, this statement of yours is a moral imperative. You are saying we OUGHT to do this. Why? Sez who? From where—in a mindless, purposeless universe—does such a moral imperative come?
“I’d be happy to elaborate on why other moral actions are better than others if you want, but none of them require submitting to a totalitarian leader.”
You mean which choices OUGHT to be made? By what standard do you determine this?
“Reason and logic [determine what choices ought to be made].”
Whose “reason”? All sorts of moral evils are committed by those who believe they have a rational, justifiable reason to do such things.
(Reason is predicated upon first principles. A person may use the exact same reason/logic as another…and arrive at vastly different conclusions if they think and reason according to disparate first principles. We see this played out every day when equally rational/logical folks—with different philosophies and worldviews—clash [we often refer to such clashes as “culture wars”].)
"What morally good action can someone who believes in god make that someone who does not can't?"
Your question has two assumptions:
1) You assume that an action can be deemed “morally good” but you have no objective, authoritative standard by which to determine what is “morally good” and “morally evil.” (Thus far you’ve offered your opinions and preferences and nothing more.)
2) You assume that “morally good action” involves horizontal relations but not vertical. But Jesus is clear: “Jesus said to him, ‘You shall love the Lord your God with all your heart, with all your soul, and with all your mind.’ This is the first and great commandment” (Matthew 22:37-38).
Thus, according to Jesus Christ, the atheist cannot obey the FIRST and GREAT commandment; viz. the atheist cannot love God.
Again, according to Jesus Christ, to love God is morally good. And this, by definition, the atheist cannot do.