Midrash su Deuteronomio 20:18

Our Rabbis were taught: How did the Israelites write the Torah? R. Juda said: "Upon stones did they describe it as it is said (Deut. 27, 8) And thou shalt write upon the stone all the words of the Torah, and afterwards it was covered with plaster." R. Simon then said to him: "According to your opinion how were then the nations able to study the Torah from the stones?" Whereupon he replied: "The Holy One, praised be He, gave the nations exceptional wisdom so that they sent their representatives who removed the plaster from the stones and copied the Torah from it. In that moment, the nation's decree was issued in punishment for their not having studied the Torah." R. Simon said: "The Torah was written upon plaster, and below it is inscribed (Ib. 20, 18) hi order that they may not teach you to do in accordance with all their abominations. From this it may be inferred that if the nations will repent, they are to be accepted." Raba b. R. Shila said: "What is the reason of R. Simon's theory [that it was written upon plaster]? Because it is written (Is. 33, 12) And the people shall be burnt as lime; i.e., on account of the Torah which was written on lime." But R. Juda will explain that this means, just as there is no remedy for lime, except fire, so also will there be no salvation [with those nations who do not adhere to the Torah], except burning. In accordance with whom will the following Baraitha agree? (Deut. 49, 10) And thou takest captives of them. This includes also the Canaanites residing outside the land of Israel, who if desirous to return in repentance that they should be accepted. (Fol. 36) The above Baraitha is in accordance with the opinion of R. Simon.

(Devarim 20:18) "So that they not teach you to do according to all their abominations": whereby we are taught if they repented, they are not killed.