As to the targeting of individuals: The purpose is to reduce enemy capabilities and render them as harmless as possible. How much does his or her death advance that cause?
Operative example: This is World War Two; You are an OSS sniper in Germany. You are at a public event. On the stage are Hitler himself, Heydrich, his even more thuggish heir and a decent soldier named Erwin Rommel. You can get off two quick rounds, so of the three whom do you shoot? Answer you shoot Rommel twice just to make sure. Fuherers and their heirs are a dime a dozen. First-rate Field Marshalls are hard to find. End of lesson.