I am prior service and I just got the chance to catch up with a friend of mine who joined the service with me. My friend joined Special Forces and has been pretty active. He told me that he has seen a lot of things and has been in some pretty bad situations in various places. I am a Christian, but my friend is an atheist. Well, actually he doesn't give it much thought nor does he care. One day I asked him if he believed in God and eternal life. He replied that it did not matter, because either way he just did not care. I asked him if he believed in Jesus? He replied that religion did not matter to him. "What about morality"? I asked, he said that his morality depend on the time and place and that what ever completes the mission is what needs to be done. That kind of scared me. My friend is now a contractor and goes overseas regularly. The point of this post is to ask what is an atheist, that is secular humanist who cares about morality and Christian to do against this sort of nihilistic thinking or lack thereof? A logical argument is not going to work on him.
Our morality is mostly innate. He may not have though consciously about moral values and such. He also may not feel any need to chase after afterlife fantasies via ancient books of fables.
One has to have some moral foundation to function in our society. He seems to be functional, so maybe there's nothing to worry about. Does he subscribe to the golden rule or some variation thereof?
If he's going off and killing for profit or whatever, then maybe being in the military has sabotaged him a bit. That happens. If this is a case, I don't see this as an issue that has much to do with religious belief.
Finally, in your post, I don't know who "an atheist, that is secular humanist who cares about morality" refers to. This forum has postings from believers of all stripes.
When I was in the Marines I met a lot of people like this guy. I was like them to a point. If you have ever watched Generation Kill on HBO that is the mentality. A lot of these guys would kill you and not even care of the consequences. May be the world is just divided in what Lt. Col. Dave Grossman refers to as Sheep, Sheepdogs, and Wolves. Honestly, I am not the most morally minded person out there, although I am not killing people left and right. But I do not care either way. Some people are cowards who hide behind traditions and fabricated moral codes to protect themselves and are unwilling to live in a chaotic environment. I have been to Somalia and honestly I would not want to live there. If you're not worried about self-preservation or others, then morality does not seem to be a big deal. If an atheist claims that it does, then what distinguishes the atheist from the dogmatic theist?