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.
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.