Oh please. Not Sacking the city and selling the children into slavery (after taking Yefas Toars and killing all the men) is a mordern concept. Everyone knows Chadash is Assur Min Hatorah!
(End Troll).
Seriously, many of the laws are to encourage interaction between Jews, and discourage close interaction with Aino Yehudim. As far as exterminating the 7 nations & Amalek, sof davar answered well. We offered the 7 the opportunity to leave (which some took), which is better than they offered those before them at the time of Avraham.
Amalek has been discussed here many times, I will not beat a dead horse.