Two wrongs make a right

Commonly used to excuse capital punishment: execution is okay because a murderer is being killed.

(Which, by presupposing that "execution" is a wrong, provides a splendid example of the logical fallacy known as "Begging the Question.")

A better statement would be this:

Given that killing is accepted to be usually wrong:

Killing as a punishment is acceptable if the criminal is a killer.

This fallacy is often committed by children. An example:

Parent: Jim, why did you pull your sister's hair? Don't you know that's wrong?

Jim: I know, but she pinched me first.