I believe that we would lower ourselves to the level of the monster who lived to inflict pain on others.
I always think of it as shooting a rabid dog: put the poor thing out of its misery.
Let our Creator do the punishing.
He will, too, won't He? He always does.
When it comes to my daughter, If I catch them in the act I will be lower than a monster. Like you said...God will judge, I will just get him to the judgement as painfully as I can.