What are the ramifications of "Lo Sechayeh Kol Neshamah"?
Ramban: Refer to 20:10:2:1*.
Rashbam: Once Yisrael come to fight with the Cana'ani nations, it is a La'av against offering the Cana'ani nations peace-terms. 1
Rashbam: If however, before they go into battle, they come to make peace, as the Giv'onim did, one is permitted to accept them. See also Ramban on Pasuk 11, DH 've'Da'.
Having already commanded, in Mishpatim, Sh'mos, 23:33 "Lo Yeishvu be'Artz'cha Pen Yachti'u oscha Li", why does the Torah se fit to repeat here "Lo Sechayeh Kol Neshamah"?
Ramban (on Pasuk 18): Whereas the Pasuk in Mishpatim is referring to the seven Cana'ani nations at large, the Pasuk here is coming to add a La'av to allow even individual Cana'anim to remain alive to serve us, in case they teach us how to worship their idols.