What do you call a person who advocates the use, against an enemy, of methods borrowed from that enemy, or applies the same moral standards as the enemy does or otherwise copies the enemy's behavior or techniques?
A judo throw uses the opponent's momentum against him.
From Shakespeare's Hamlet, the phrase hoisted by his own petard has entered into common usage.
There's letters seal'd: and my two schoolfellows, Whom I will trust as I will adders fang'd, They bear the mandate; they must sweep my way And marshal me to knavery. Let it work; For 'tis the sport to have the enginer Hoist with his own petar; and 't shall go hard
The meaning is less like using the enemy's techniques against him and more like the enemy being caught with his own techniques.
Fighting fire with fire is another phrase.
Sorry, I don't have any single-word phrases, but perhaps one of these colorful metaphors will do.