Oh shit you're right, better just let him go so he can murder more people. Maybe next time he'll nuke two cities.
We wouldn't want to ever have a spine would we? We need to be unrealistically lawful good all the time 100% of the time, or else it's just a slippery slope until we're exactly like the Joker! We need to just sit around twiddling our thumbs and whistling, hoping that by setting an example of blase passivity that maybe the villains will run out of people to murder and eventually stop killing people.