Do atheists even need an objective moral system?

[Tea Earl Grey Hot]

...
Taking your idea to its logical conclusion - I like apple pie, but hate cherry and I like Swiss Cheese but hate goat cheese. So I can just say that because I don't like those things I want to make them illegal and if I get enough people on my side, I should succeed in doing so. If you happen to like Cherry Pie or Goat Cheese - tough luck.

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. )

Are those values arbitrary? Or are they determined by the nature of morality?

A moral system tells a person how to live his life. Logically, living is a value that is not only consistent with it, but required for it as well. Any additional values may be judged by how well they promote life. For example, a totalitarian, utopian society can be considered as harmonious, but a place where any and all dissidents are killed is not conducive to life. Therefore, we value peace and harmony to the extent it makes life possible and better, but once it starts doing the opposite, like in oppressive regimes, we consider violence and chaos to be the better alternative.

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.

Consider the parallel with science. Science tells you how nature works and technology is the application of scientific principles towards the goals of our choosing. Does the fact that we can choose what to apply those principles towards make science subjective? Are any and all technological goals equally arbitrary irrespective of whether they conform to the scientific concepts? ...

Science is the study of reality. People do science because they value better understanding reality. If one did not value the pursuit of understanding nature, then they would not be into science. We can call somebody a "good" or "bad" scientist based on how well they investigate reality in way that is conducive to better understanding the way reality really is.

Technological goals are ultimately arbitrary. Stopping global warming is an arbitrary goal because it's based on the value that places importance on the continued existence of mankind. But I can't see why we should ultimately care about human existence. We could just as easily say "fuck it all" and let the Earth turn into Venus.

Similarly, the legitimacy of our moral and value systems is determined by the degree of consistency.

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