When does God condone killing?

Despite the OT Commandments "thou shalt not kill", God seems to have condoned killing, as in the battle of David versus Goliath, where David is presumed to have Gods Blessing. In the NT, while Jesus does not directly speak out on one person killing another, He does caution "Live by the sword, Die by the sword" and "turn the other cheek".

Under what conditions do you think God would consider one person taking the life of another exempt from the commandment "Thou shalt not kill"? Consider the following situations: A soldier in battle; A policeman on duty; A father defending his family from intruders; A woman defending herself in an attempted rape.