Sefer HaChinuch ספר החינוך

To not make a covenant with the seven nations and likewise with any worshiper of idolatry: That we not make a covenant, meaning to say that we promise our friendship to the bad nation of heretics — which are the seven nations in the Torah which were holding our Land before our arriving there, and they are the Hittite, the Emorite, etc. — as it is stated (Exodus 23:32), “You shall not make a covenant with them and their gods.” [This] means to say that we should not make peace with them and leave them to worship idolatry.

It is from the roots of this commandment to destroy idolatry and all of those who serve it from the world. And these seven nations were the center of idolatry and its first foundation; and because of this, they were uprooted from their Land. And we were commanded to uproot them and to destroy their memory forever. And it is as it is written about them in the Torah (Deuteronomy 7:2), “and you shall surely annihilate them” - and that is a positive commandment in the Order of Va’etchanan. And there I will write at length about this commandment, with God’s help, and tell the reason for why these bad nations are in the world and that this commandment is included in the commandments that are practiced. And from the warning about them, we understand a warning not to make a covenant with any worshipers of idolatry. But there is a distinction between the seven nations and the other peoples that are worshipers of idolatry (Gittin 45a); as there is no commandment to kill the other peoples if they do not fight with us, but these seven nations, we are commanded to kill in any place that we are able to — unless they abandon their idolatry. And the matter is because they were the center of idolatry and its first foundation, as I have written. And anyone who has one of them come to his hand and he can kill him without danger, but does not [do so], has transgressed a negative commandment. And that which we said that we do not kill worshipers of idolatry when they are not fighting with us, is specifically [regarding] worshipers of idolatry from the [other] peoples. However it is a commandment upon us to kill an Israelite that worships idolatry — such as the sectarians, the apostates and the heretics — because they oppose Israel. And it is better that a thousand like these be destroyed than one proper Israelite.

From the laws of the commandment — that which they said that we accept them if they want to return and repent; and the rest of its details — are elucidated in Sanhedrin. (See Tur, Yoreh Deah 191.)