Do atheists even need an objective moral system?

[Tea Earl Grey Hot]

I guess I come from the point of view that, if you want to use force/violence to violate someone’s will, it is your burden of proof to justify your action.

That's just your opinion. I could be of the view that you need no proof at all to justify action. See, it's easy:

I guess I come from the point of view that, if you want to use force/violence to violate someone’s will, you don't need a burden of proof to justify your action.

Rape and slavery should be considered unjustified by default just as God should not be assumed to exist by default. The person claiming that God exists has a burden of proof to fulfill and so does the moron who claims that rape or slavery are justified. ...

The above is just assertions. You need a 'why' to all those 'shoulds' and 'burden of proofs.'

All that is required for a rudimentary objective morality are empathy and reason.

Or maybe it's malice and primal carnality? Or maybe it's a love of polka dot underwear and bunny slippers?

I think an important distinction to make here is objective vs subjective and relative vs absolute. Morality can be both objective an relative. For example, it could depend on the circumstances involved but be independent of personal tastes.

Yes, but what I'm talking about are the fundamental values upon which you build your moral system. To me it all seems completely arbitrary.

But how are moral nihilists any more justified in their position than theists are? I mean, even a theist that acknowledges that God does not exist could just say that they just personally prefer fiction over truth the same way someone might prefer cheddar over Swiss or rape over consent.

Yes. You can say you prefer fiction over truth if you want. I'm not stopping anyone who wants that. But I'm pretty sure that a good number of us prefer truth over fiction so under that preference for truth we can get to the bottom of whether morality is like truth.