The gemara (חולין יא) discusses an unusual scenario in Jewish jurisprudence known as: "לא הרגו נהרגין, הרגו אין נהרגין". The situation as described by the Meforshim is when witnesses who testified about a murder are later proven false by later testimony which places them elsewhere at the time that the alleged murder occurred (עדים זוממין), then these false witnesses are executed in place of the person they tried to have killed. However, if the accused had already been executed for a crime, even if the witnesses are later proven to have lied, then the false witnesses are free from this reciprocal punishment and they are not killed by the hand of Beis Din. This is derived from the words in the posuk, "ועשיתם לו כאשר זמם לעשות לאחיו" - "It shall be done to him what he had planned to do to his brother," only what "he had planned to do" but not if he had actually done it.

     R' Dovid Feinstein zt"l (Kol Dodi) writes that although it does appear unusual that the false witnesses can have another man executed unjustly and they do not suffer a similar punishment, however, upon closer analysis, we can understand the Torah's justification for this rule. Hashem, Who is just in all His ways, would not have allowed the accused to be executed unless that person actually deserved that punishment. Therefore the false witnesses, even though they did wrong, did not cause an unjust death and so do not deserve execution.

     On the other hand, if their conspiracy is uncovered before the accused is executed, we know that Hashem saved him because he did not deserve death. Thus, it turns out that these false witnesses tried to have an innocent man killed, and for this they are themselves executed.