The point is to make it clear that the use of WMD will be a reason to be destroyed. IF we did not show that the end result makes the use of WMD not practical, they would continue to be used. Wishing them away does not work.

A 250 KT device would probably kill as many as , say 20 million people, in someplace like Hong Kong or Mexico City. Lots more injured. The whole idea is to insure that whoever set it off does not have the chance to do it again. One strike, and you're out.