If I kill a murderer, a maniac, a bastard for his murders, will I become like him? By the idea, I would also become a murderer, but I had other goals - in retaliation, to make the world better. On the other hand, he may have exactly the same motives when he killed certain types of people. This is a moral paradox. How to get out of it?
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Without a joke, this paradox was well demonstrated in the Batman comics. If you kill a murderer, you become a murderer because you put your judgment and your motives above public goals. The main objective of the ideal judicial system is not only to punish a particular offender, but also to show to everyone the inevitability of punishment and thus prevent the commission of new crimes. And if you extinguish a maniac in a dark basement, then no one will know why he is so. So it's better to act like Batman - to give the maniac to the organs, having supplied all the evidence and pre-breaking him all the bones so that he doesn't joke anymore.