Do the 10 Cs Only Apply To Our Dealings With Other Christians?

I must say I am a bit puzzled by this Scripture.  Does the 10 Commandments apply to all of our dealings with others or just our dealings with other Christians?  After reading today's passage I'm not sure what to think.

1 Samuel 27:8-12(NKJV)

8And David and his men went up and raided the Geshurites, the £Girzites, and the Amalekites. For those nations were the inhabitants of the land from of old, as you go to Shur, even as far as the land of Egypt. 

9Whenever David attacked the land, he left neither man nor woman alive, but took away the sheep, the oxen, the donkeys, the camels, and the apparel, and returned and came to Achish. 

10Then Achish would say, “Where have you made a raid today?” And David would say, “Against the southern area of Judah, or against the southern area of the Jerahmeelites, or against the southern area of the Kenites.” 

11David would save neither man nor woman alive, to bring news to Gath, saying, “Lest they should inform on us, saying, ‘Thus David did.’” And thus was his behavior all the time he dwelt in the country of the Philistines. 

12So Achish believed David, saying, “He has made his people Israel utterly abhor him; therefore he will be my servant forever.”

OK, here we have David saying that they go on raiding parties to other countries where they kill all of the human inhabitants and take all of their belongings. 

Then when David gets back and the Philistine king Achish asks where David has been David lies.  And Scripture tells us that the reason that David kills every living human being is to keep his lie from being discovered.

That sounds like lying, murder and theft to me.  Now I know tings were done differently back then and life wasn't nearly as civilized as it is today but I don't find anywhere in the Bible where it says thou shalt not lie, murder, and steal; except from your enemies.

And I don't see where David is ever reprimanded or called to account for that behavior.  In fact God seems to still consider David a man after His own heart.  The first thing that we see that David does wrong is when he commits adultery with Bathsheba.   

So I am a bit stumped here.  The only conclusion that I can draw at this point is that maybe somehow the 10 Commandments don't apply to our enemies but I have a hard time believing that is the correct interpretation.  There must be something else here that I am not getting.

Oh well, the Holy Spirit will reveal the answer when I need it!