BESIDES, even if it were allowed, there would probably be one or two characters with the exceptional ability to kill everyone due to extreme mastery of their elemental bending or all bendings (the Avatar). There are things that could be considerably one-shot abilities. Such as.... well, bloodbending. Get a few people like Tarlock, where they can bloodbend without the power of the full moon, and you have a recipe to kill anyone with ease.
Not letting this happen in the show gives some sort of balance to the powers of each type of bending imo.