A million dollars is a lot more zero-sum than not killing someone - if I give you a million dollars I lose a million dollars. To make the analogy more accurate, you'd need to stipulate that Joe will kill me if I don't kill him.

Also, I don't think it's fair to ignore the fact that for most people, not killing someone is vastly easier to do at non-self-destructive costs. I appreciate that this is a quantitative argument rather than a categorical counterargument, but if we have atheists who base their sense of morality on a vague consequentialism that they can't quite fully articulate, that's still no worse than Robertson's (presumed) divine command theory, and they should be able to make such such arguments without being accused of hypocrisy for not also advocating actions that <i>would</i> score much worse under their vague consequentialism.