Mishnayos Makkos Perek 1 Mishnah 6

The conspiring witnesses are executed only if they are rendered conspiring witnesses after the verdict of the accused is concluded. This is in contrast to the opinion of the Sadducees, as the Sadducees say: Conspiring witnesses are executed only if they are rendered conspiring witnesses after the accused is killed on the basis of their testimony, as it is stated: “A life for a life” (Exodus 21:23; see Deuteronomy 19:21). The Rabbis said to the Sadducees: But wasn’t it already stated: “And you shall do to him as he conspired to do to his brother” (Deuteronomy 19:19), and this latter verse indicates that his accused brother is alive? And if so, why is it stated: “A life for a life”? One might have thought that if they are rendered conspiring witnesses from the moment the judges accepted their testimony in court, they will be executed, even though no verdict was concluded. Therefore, the verse states: “A life for a life,” teaching that they are executed only if they are rendered conspiring witnesses after the verdict of the accused will be concluded, from the moment that the court is on the verge of taking his life.

אֵין הָעֵדִים זוֹמְמִין נֶהֱרָגִין, עַד שֶׁיִּגָּמֵר הַדִּין, שֶׁהֲרֵי הַצְּדוֹקִין אוֹמְרִים, עַד שֶׁיֵּהָרֵג, שֶׁנֶּאֱמַר נֶפֶשׁ תַּחַת נָפֶשׁ. אָמְרוּ לָהֶם חֲכָמִים, וַהֲלֹא כְבָר נֶאֱמַר (דברים יט) וַעֲשִׂיתֶם לוֹ כַּאֲשֶׁר זָמַם לַעֲשׂוֹת לְאָחִיו, וַהֲרֵי אָחִיו קַיָּם. וְאִם כֵּן לָמָּה נֶאֱמַר נֶפֶשׁ תַּחַת נָפֶשׁ, יָכוֹל מִשָּׁעָה שֶׁקִּבְּלוּ עֵדוּתָן יֵהָרֵגוּ, תַּלְמוּד לוֹמַר, נֶפֶשׁ תַּחַת נָפֶשׁ, הָא אֵינָן נֶהֱרָגִין עַד שֶׁיִּגָּמֵר הַדִּין: