Imagine that one month after the 9/11 attacks the U.S. conducted a massive bombing campaign against terrorist training camps in Afghanistan. Further assume that (1) the U.S. knew Bin Laden was well hidden and so the attacks would have no significant chance of killing him, (2) the U.S. military had estimated that about 5,000 innocents would die because of the bombings and indeed around 5,000 innocents did die, and (3) the bombings were conducted mainly to deter future terrorist attacks against the U.S. Such a bombing campaign would have been widely supported in the U.S. Most Americans would assign the moral blame for the civilian deaths to Bin Laden.
But now imagine that instead of conducting a bombing campaign President Bush ordered the assassination of Bin Laden’s closest relatives. Assume that 500 people were killed including all of Bin Laden’s parents, grandparents, children, uncles, aunts, and first cousins who were alive on 9/11. President Bush, lets assume, announced that although those killed were innocents their deaths were necessary to deter future terrorist attacks against the U.S. My guess is that for ordering such assassinations Bush would have been impeached, removed from office and sentenced to either death or life in prison.
The relative expected responses to the bombing campaign and assassinations seem inconsistent because fewer innocents would die with the assassinations and the assassinations might well have a greater deterrence effect on future terrorists. We would be less bothered by the bombings, I suspect, because we could claim that we didn’t want the innocents to die. But if we estimate that X number of innocents will die because of a military strike, why should it morally matter whether we actually wanted these X people to perish?