Do atheists even need an objective moral system?

[Tea Earl Grey Hot]

Yes. Nothing really stopping you from trying. Though I don't like being told how the eat so I will try to stop your efforts just like the rapist will keep on trying to rape.

(I know that sounded terrible. )

So, your position is "might makes right", then? Those in power get to dictate morality? Then that would make "power" a non-arbitrary moral value.

No, not at all. Might is just might. Weak is just weak. Rightness or wrongness is not connected to the amount of power of an individual.

A despot ruler is "right" to murder his subjects but his subjects who hate his rule are also "right" to try to overthrow the despot.

Perhaps for some people, a life where dissidents are not killed is a life not worth living? Or perhaps a person only values their life and hates every one else's. If said person had the magic ability to kill everyone to leave the world for himself, is that truly bad? He's merely making life "better" for himself.

Is his life better? Really? Better or worse here are not just subjective opinions - they are judgments grounded in the reality of human nature. It is not an "opinion" that a rich man with a secure future and material comforts is living a better life than a poor man living hand-to-mouth. It does not matter if one stops valuing things that make life better - their significance to life does not decrease. It doesn't matter if you don't value food - you still have to eat or your life won't be "better".

Perhaps some people like living as a bum? You're conflating common ideas of better or worse with real better and worse. The two are not the same.

If I don't eat, I starve. But who's to say that's worse? It's worse for me because like to live and I like to eat, and I don't like pain and suffering. But if I were some suicidal masochist, that might be a fun time.

You don't choose your values in a vacuum. You don't wake up one day and arbitrarily decide on a list of things you are going to value from this point onwards.

True. But the values I end up with are not identical to anyone else's. My core values are rather normal but you can easily find people throughout history with apparently sadistic and selfish values.

And the values you choose also have to pass the muster of objective morality. For example, you can choose to believe that a life where dissidents survive or where anyone else survives is not worth living - but those chosen values would be immoral themselves.

Well this just came out of nowhere. Looks like we're back at square one.

Let's just set up an example: I have enormously powerful nuke that could destroy the world killing everyone. Why would it be wrong for me to use it?

In this analogy, I'm equating science to morality - a certain set of concepts and principles. Technology, then, is similar to our value system - goals to be achieved by the application of those principles.

First of all, those goals cannot be chosen arbitrarily. The first test they need to pass is conformity to the principles in question. Which is why, my technological goal of turning water into wine is unscientific and my value of killing all dissidents is immoral. Secondly, even if we chose the goals arbitrarily form a set of goals that to conform, that does not make the principles to be applied subjective or arbitrary.

Give me some examples of these principles.

And if somebody does not value consistency? Perhaps for some, inconsistency (or the seeming appearence of inconsistency) is what makes a morality "legitimate"?

Irrelevant. Objective morality does not depend upon any individual's personal wishes or values. It is his values that are to judged and changed accordingly.

This is just an assertion. It's not clear to me how nuclear annihilation is any more "wrong" than mixing plaid and stripes.