What IS good, and how do we determine it?

[Mister Agenda]

It would obviously be wrong via the wife's feelings at being murdered by the man whom she thought loved her above all else.  

Not to mention that killing of one's own clan in primitive times was considered wrong due to the fact that survival was of the up-most importance; thus morality was born.

But if the man thought that it was GOOD to kill her so that he could marry someone else that he was interested in, would THAT be good from his perspective?

Are you familiar with the fallacy of equivocation? It's when you use a word that has mutliple senses in a deceptive or confusing way. For example, in your sentence, it's unclear whether you mean 'morally good' or 'in the man's best interests'; or whether you mean it in one sense the first time and a different sense the second time.

Bear in mind that not all atheists agree on moral issues (and neither do all Christians or even all Catholics). 'The man' was wrong in thinking it was morally good to kill his wife, but he may have been correct in thinking that it would work out to his advantage; especially if he is a sociopath incapable of being damaged by emotional consequences for his act. But there's no system of morality that can prevent sociopaths from being sociopaths, is there?