1. Destructive nuclear capacity exceeding the amount needed to destroy an enemy.
2. Excessive killing.
3. An excess of what is necessary or appropriate for a particular end: "government overkill in dealing with dissent".
4. Moron gunstore sales clerk spending way too much valuable time and energy worrying that a gun might somehow be too big to kill a deer with.
So as long as you dont use a nuke, patriot missile, mortar, RPG, or kill more animals then intended it would not be overkill