Criminology

In criminal profiling, overkill refers to the agressions done by an offender (specially a serial killer) to satisfy his fantasies, and that are not necessary to kill the victim. They are often carried-out post mortem.

Nuclear application

Overkill is especially used to refer to a destructive nuclear capacity exceeding the amount needed to destroy an enemy.

General application

The term is often used in situations when a person spends a large amount of time or money on a task or component, when a lesser alternative would work just as well. For example, buying 7 mm thick, high quality audio cables to connect a computer to a pair of poor-quality speakers would be overkill.