Evil Atheists

[DaveD]

If I were to meet an atheist who presents as a happy, confident, solvent individual, beautiful family, good friends etc (you get the picture) who went on to tell me the secret of his success in life was to lie, cheat, steal and stab others in the back at every opportunity, take no prisoners and give no quarter, be ruthless with strangers and only protect family and loved ones. How could I tell him that his behavior was "wrong" or "evil" using an atheistic worldview? "The tears of strangers are only water" would be an apt motto for this individual.

All I would say to your "friend" is that one day he will be the victim of someone lying, cheating, stealing and stabbing. That his behavior becomes a model to be emulated by others, and does he want to really live in a world in which everyone else is lying, cheating, stealing and stabbing?

At last an attempt to answer the question. He would simply respond by saying that your making the assumption that the welfare of others makes any difference to him or his family, he's quite happy living in that kind of world, thats how it is, survival of the fittest, why should he care about the welfare of others, they are merely evolved machines just trying to survive, of no consequence to his well being or his families. Nothing wrong with that is there?

Nothing wrong at all, apart from the fact that it's complete and utter bollocks.
That we are "evolved machines just trying to survive" isn't a problem, but the addition of "merely" is. The whole is greater than the sum of its parts.
One of the means of survival is cooperation, something your hypothetical atheist seems incapable of. He sounds as ignorant as any theist, and is very atypical. In fact, I would advise him to skip to the end of L. Frank Baum's story immediately and acquire a brain, for his own good.