See also: One of the enemy.

Examples:
* Using same methods as the enemy, especially when those methods are reviled (e.g. using "dark" magic to defeat practitioners of "dark" magic)
* Becoming one with the enemy to defeat them, such as by creating a hybrid or turning one of their kind against them (somewhat common with demons and robots; does not include infiltrating their ranks)

This is commonly seen as having a "price" that's not worth paying even if it brings victory. Occasionally, especially in darker fiction, has the victory turn into some form of semi-defeat instead (e.g. they become the same as their enemy in the process).

Nuclear war can possibly be considered to be part of this, but should not in most cases.

Simply using same methods, weapons, etc. should NOT be sufficient for this tag.