The Allies won World War 2 largely by killing about 2 to 4 million civilians in Germany and Japan. Therefore, it isn't clear that the benefits of not killing civilians far outweigh the costs.

This will become more important as technology decreases the difficulty of building WMDs. Eventually, even a small nation like North Korea will be able to make nuclear missiles. By that time, the cost of allowing them to do as they please (and encouraging other nations to also do as they please) may be greater, in expected lives lost, than the cost of brutally killing a million North Korean civilians.

I would go on, but there's no point in going to the next shock level.