As far as I am able to tell, overkill was first introduced as a term used in nuclear weapons strategy. Any weapon of mass destruction, when used on a populated area (heaven forbid) will kill far fewer people than it could!

The reason is simple: the amount of energy released is sufficient to vapourise (literally, unfortunately) a huge number of people. But there are not that many people in the blast radius. Hence overkill: the excess energy released in the region.

Yet another reason why nuclear weapons should never be used.