The short answer is: No, it was not the norm to kill men and women and children in war.
The standard practice in ancient warfare was to offer peace when besieging a city, of course the peace terms were conditioned on complete surrender of the inhabitants and by reducing it to a vassal kingdom. If they didn't accept complete surrender, the male population was slaughtered and the women and children sold into slavery. That was the accepted practice in the ANE, though there were exceptions to the rule, see Ancient siege warfare by Paul B. Kern p. 68-76. Indeed this is exactly what we find in Deuteronomy chapter 20 (except for the Canaanite cities which all the inhabitants were to be slaughtered), and it undoubtedly reflects the reality of warfare in ancient Israel.
The brutality of David's actions directed against the Geshurites, the Girzites, and the Amalekites were not the norm in Israel but rather an anomaly. As the bible itself is aware, there was a reason why David was so harsh with them. Verse 11 explains that these tribes were allied with the Philistines and if he were to keep them alive they would report to Achish his lord, which would certainly jeopardize his life, he deemed them a threat and justified a mass genocide. The fact that the author of Samuel felt the need to include an explanation reveals that it was not the accepted practice in ancient Israel, an unusual behavior that required an explanation, and may even have "viewed this behavior as embarrassing, unusual, unreasonable, or otherwise reflecting poorly on David." See also 1 Chronicles 22:8 for more evidence on how the biblical authors may have viewed David's brutality.
This may or may not justify the brutality of David's actions and its not my goal here to vindicate David, i'm just trying to put things in perspective. David did not treat all his captives like that (see 2 Samuel 8. He conquered Aram and Moab and didn't even kill all the males but put them to good use in his kingdom), and there is no evidence that this was the accepted practice in the times of David, if we take the bible at face value then David did so only when he was forced to given the circumstances he found himself in. Indeed the Assyrians known for their psychological warfare/tactics, when they wanted to instill fear in the hearts of the nations they didn't discriminate between males and females but killed and burnt them all, and tortured their captives in the most horrific ways see this. David himself employed similar psychological tactics in his war against the Ammonites, see 2 Samuel 12:31. So exceptions to the standard rule were common in ancient warfare, and David was not any different when it came to warfare, he killed men and women when he felt that it would ensure his safety as was the case in this particular instance.