Learn English – Word for opposite of *prerequisite*? Something that is possible because of another thing

There are a few things that could apply here.

You could be looking for the word consensus:

Consensus
1 a : general agreement : unanimity b : the judgment arrived at by most of those concerned 2 : group solidarity in sentiment and belief

Or you could be looking for the logical fallacy Argumentum ad populum:

Argumentum ad populum
In logic, an argumentum ad populum (Latin for "appeal to the people") is a fallacious argument that concludes a proposition to be true because many or most people believe it. In other words, the basic idea of the argument is: "If many believe so, it is so."

This type of argument is known by several names,1 including appeal to the masses, appeal to belief, appeal to the majority, appeal to democracy, argument by consensus, consensus fallacy, authority of the many, and bandwagon fallacy, and in Latin as argumentum ad numerum ("appeal to the number"), and consensus gentium ("agreement of the clans"). It is also the basis of a number of social phenomena, including communal reinforcement and the bandwagon effect. The Chinese proverb "three men make a tiger" concerns the same idea.

You could also find the word or phrase you're looking for in that definition.

This is called a necessary evil (also known as the lesser of two evils). It is used to refer to a choice that is bad, but preferable to the alternative choice that is much worse.

In your example, you have 2 choices:

1. Kill people who are violent criminals.
2. Do not kill those violent criminals and allow them to continue to harm society.

Killing violent criminals could be considered immoral, but it can be considered a necessary evil since the alternative of letting them live and continue to harm society is worse.

There are even a lot of references on the Internet to Dexter as a "necessary evil".