Rashi on Deuteronomy 19:19/רש"י על דברים י״ט:י״ט

1כאשר זמם [THEN SHALL YE DO UNTO HIM] AS HE HAD INTENDED TO HAVE DONE [UNTO HIS BROTHER] — as he had intended to do, but not as he has done! Hence they (the Rabbis) stated that if they (the false witnesses) have already killed (i.e. if by their evidence the defendant has already been executed) they are not put to death (Makkot 5b).

2לעשות לאחיו [THEN SHALL YE DO UNTO HIM AS HE HAD INTENDED] TO HAVE DONE TO HIS BROTHER — What is the force of לאחיו? Would it not have sufficed to state כאשר זמם לעשות? But it teaches in regard to witnesses who have been proved to have “plotted” against a married daughter of a priest by an accusation of adultery, that they are not to be punished with burning (as would have been the punishment of the accused woman, if she had been found guilty), but with the death penalty to which the alleged adulterer would have been subject, which is strangulation. For it is said of such a woman, (Leviticus 21:9) “she shall be burnt to death” — ‘‘she”, but not her paramour (he is not burnt but strangled) — therefore it states here לאחיו, “unto his brother” — ‘‘[ye shall do unto him] as he had intended to have done to his brother” but not as he had intended to have done to his sister. This case, however, is an exception, but with regard to any other death penalties Scripture puts women on the same level with men, and consequently those who are proved to have “plotted” against a woman are made to suffer the same death penalty as those who are proved to have “plotted” against a man. For instance, they have testified against her that she killed a person, or that she desecrated the Sabbath, then they are made to suffer the death she would have suffered, for Scripture excludes here by the word אחיו the “plotting” witnesses from the death penalty of the sister (i.e. which the woman would have suffered) only because it is a case where the law regarding plotting witnesses can be applied to them by letting them suffer the death-penalty of the paramour (Sifrei Devarim 190:14; Sanhedrin 90a).