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 them peace-terms. 1
See Oznayim la'Torah, who explains in detail why the Cana'anim needed to be exterminated and why Hashem commended specifically Yisrael to do it.
To whom does "Lo Sechayeh Kol Neshamah" apply"
Rashi in Sotah, 3b: It applies to the seven nations ? the Chiti, the Girgashi, the Emori, te Cana'ani, the O'rizi, the Chivi and the Yevusi.
How did they kill them?
Sanhedrin, 67a: By the sword (as the Torah writes in Bamidbar, 21:24, in connection with Sichon - Va'yakeihu Yisrael L'fi Charev". 1
See Torah Temimah, note 51.
Having already commanded, in Sh'mos, 23:33 "Lo Yeishvu be'Artzecha Pen Yachti'u oscha Li", why does the Torah need to repeat here "Lo Sechayeh Kol Neshamah"?
Ramban (in Pasuk 18): Because, whereas the Pasuk in Sh'mos is referring to the seven Cana'ani nations in general, the Pasuk here is coming to add a La'av against allowing even individual Cana'anim to remain alive to serve us, in case they prevail upon us to worship their idols.
