When you are in a fight it's the aggressor who initially sets the parameters of the fight. If you don't match those parameters and then some you are going to lose the fight. The difference is that once we have ended the fight we go back to being ourselves. If it were a matter of becoming indistinguishable from them then we would leave that fight and head out looking for another. When you decline to use whatever methods necessary to end a fight as soon as possible you are simply accepting a higher toll for the battle/war than if you had ended it quickly. Several hundred thousand people lost their lives when we used atomic bombs against Japan. The alternative was an invasion of their homeland and the lives lost would have numbered in the millions, many of the allied forces. If by ending that war as quickly as possible meant we had lowered ourselves to their level we would have used nuclear weapons in Korea. Fighting fire with fire does not make one an arsonist.