The pessukim talk about witnesses who testify in bais din to kill someone, and are found to be disproven. The passuk states ועשיתם לו כאשר זמם לעשות לאחיו which Onkelos translates as “And you shall do to him as he thought to do to his brothers.” Rashi brings from here Chazal say that we punish witnesses who plotted to kill, by giving the punishment they tried to cause to another. However, if witnesses testified against a person to kill him, and the person was indeed killed by bais din, the witnesses will not get the punishment they caused. The Mefarshim are bothered, why is this not a kal v’chomer. If witnesses plotted to kill someone and were not successful, as they were disproven, they get killed, where they testified and killed him, certainly the witnesses should be killed?
The Gemara brings that one who worships the avodah zarah of molech with one of his children is liable for the death penalty as it states כי מזרעו נתן למולך since from his children he gave over to molech. However if one worships molech with all his children, he does not get killed. This is wondrous: if a person who brings some of his children is killed, for sure someone who brings all his children should be killed as he is even worse? The Maharsha answers bshem the S’mag: one who kills gets killed by bais din in order to atone for his sin. This person who brought all his children to molech transgressed such a terrible sin that Hashem does not want him to receive an atonement here in this world, rather he will be punished the next world.
Says the Maharsha, this logic also explains why plotters are not killed if the judgment of the death penalty was executed and the person was killed. The Torah only provides atonement in this world for plotters who testified falsely to kill and are disproven before the execution of their testimony, since they did not sin so severely. However once their testimony was accepted and brought into fruition, they not only plotted to kill, but actually caused him to be killed, sinning severely. Hashem does not want them to receive atonement here in this world, rather they will be punished in the next world.
The Ramban answers differently, it states בקרב אלקים ישפוט amongst G-d will judge, and כי המשפט לאלוקים הוא since judgment is Hashem’s. Hashem is amongst the judges and will not allow something to be decided improperly. When witnesses testify against Reuven to kill him and other witnesses come to disprove the first group of witnesses, this reflects that Reuven is innocent and is not deserving of death. Hashem won’t make a rasha into a tzaddik to be innocent, he is indeed innocent. However if Reuven does get killed, then the testimony by the witnesses is considered true, and the decision made by bais din is exactly what was meant to be paskened. Since he was deserving of death for some unknown reason, Hashem allowed him to be killed, or else he would not have been killed. The judges of Yisrael have a guarantee from Hashem that what they pasken was accepted.
This idea is hinted in the words אשר להם הריב לפני ה’ which Onkelos translates as “That they have judgment before Hashem.” The Ramban explains, when witnesses come to bais din in front of the judges, they stand before Hashem who will lead them to the truth.