Ahh, 'Black Button' I thought that was a great little film, it's not really all that deep, though. I'm surprised to see you guys wondering.

The moral of the story is that the 10 million is always worthless cause he's already dead, so the choice is between nothing and nothing, the "devil" never has any power at all, Roberts holds all the power. Ultimately it comes down to what Roberts is willing to do and what he is not willing to do, will he kill if it's made ridiculously easy for him to do so. That's why there is the little dialogue about accidents, people dying everyday and overpopulation, that part is not about the money, that part is about Roberts bringing out the rhetoric against his own good judgement. As people often do.

edit:

Also you asked - Why is Mr Roberts the antagonist.

I presume you mean to refer to his religious standing by that, yes? And that you're asking what about him infers (theologically) he should be in that position. The answer to that is - Nothing. Mr Roberts is nobody in particular, there's no subtle theistic clue as to his denomination in the film, in negative or positive terms, he's just anyone, he died and he faces a situation which judges his character.

If you want my opinion as someone culturally close to the filmmakers (I am theist and Australian) the message they are trying to get across is a personal concept of morality which is not a religious morality but a basic human morality.

The title object is the central theme of the film, it's just a button but it represents a crucial moral decision for Mr Roberts, so the point there is that humans can stand very distant from their moral responsibility, but does being far removed from the consequences make it OK to press the button?

I think heaven and hell are only employed in this story as colloquial extension props to emphasise the imperative of the central message. The central message, however, is the button, the distance a person can stand from the consequences, upon others, of their actions.