Even going back to biblical terms you can find the phrase "eye for eye, tooth for tooth" etc in many versions of the testaments including Exodus 21:24
These phrases are perhaps the first accounts of Lex Talionis, the law of like for like.
Or perhaps the basic do unto others as you would have them do unto you as distilled from Jesus' words in Sermon on the Mount.
The Golden Rule as found in many religions or texts.
In the Qur'an states: And We ordained for them therein a life for a life, an eye for an eye, a nose for a nose, an ear for an ear, a tooth for a tooth, and for wounds is legal retribution.

There definitely does seem to be precedent and even commands that transcend the laws of men (for those who are believers in that regard).

And we consider ourselves enlightened to allow these monsters to continue to exist in our world?

Where is Kryptonian technology like the Phantom Zone when you need it? Even that would probably be considered cruel and unusual punishment in today's society.

Nope better to let them roam free and slap them on the wrists each time they take a life or ruin a life. Because we can't be mean to anybody now can we? After all we might offend them or hurt their feelings.