Or HaChaim on Deuteronomy 19:19/אור החיים על דברים י״ט:י״ט

1 ועשיתם לו כאשר זמם, "and you will do to him in accordance with what he had planned to do." Our sages in Makkot 5 derive from this word that if the party falsely accused had already been executed on the strength of this fabricated testimony, the court cannot execute the witnesses. This seems a problematical exegesis. Perhaps what the Torah wanted here by writing the word כאשר זמם was to incriminate the false witnesses for merely planning to testify falsely even if they had not yet done so before a court? Once the victim of such testimony has been executed the witnesses had become guilty not only of כאשר זמם, "as he [they, as the singular is used by the Torah for a halachically valid witness, i.e. a minimum of two witnesses. Ed.] had planned," but also of כאשר עשה, "as he had done!" In that case one could not use the argument that we do not condemn somebody to be executed based only on a logical argument, i.e. if we execute them even if they did not succeed with their testimony it is only logical that we must execute them if they did succeed with the intention behind their false testimony. In the scenario we presented this principle of אין עונשין מן הדין would not apply and therefore would not stop us from executing these witnesses.

2However, the fact is that the principal exegetical argument is derived from the fact that the Torah should have written כאשר ענה על אחיו, in which event it would have referred to what the Torah wrote in verse 18 שקר ענה באחיו. Had the Torah written these words I would have known that it intended to do to the witnesses as they had done. The fact that the Torah does not use the more comprehensive word ענה, but writes the word זמם shows that it wishes to limit the parameters which enable the court to apply the death penalty to these witnesses. We have a Baraitha in Makkot 5 which goes as follows: "The son of the scholar who interpreted that the word כאשר זמם means that we do not execute the false witnesses if their victim had already been executed as a result of their testimony, said to his father: 'father, why should they not be executed? We have logic on our side; if we execute such witnesses even if their testimony did not result in their victim's death then surely we must exceute them when it did result in their victim's death!' His father replied that the sages taught that אין עונשין מן הדין, that logic does not suffice as a reason to apply a death penalty in situations where the Torah did not specifically request that a death penalty be imposed. This is not the case here." You must not deduce from this that but for the ruling that in order to apply a death penalty the Torah must state so specifically, the logic of the son would have been unassailable and I would have executed these עדים זוממים, "spurious witneses." The reason is that the distinction between the Torah writing זמם instead of ענה would not suffice to cancel the strength of the logical argument cited in the Baraitha. This is why we also needed the principle of אין עונשין מן הדין, in order not to arrive at a faulty ruling.