The "conquest" of Canaan apparently did not happen the way the OT tells us. Those accounts are only a later rationalization/reconstruction of the nation's past, ideologically and religiously overloaded. Many scholars think that what the OT purports as a violents conquest was in reality an almost peaceful, gradual settlement of the Jewish tribes in Canaan (see for instance Karl GNUSE, Israelite Settlement in Canaan: A Peaceful Internal Process, in «Biblical Theology Bulletin» 21 (1991), pp. 56-66, 109-117; GöstaW. ÄHLSTRÖM, Who Were the Israelites? , Winona Lake, Eisenbrauns 1986; Robert B. COOTE, Keith W. WHITELAM, The Emergence of Early Israel in Historical Perspective, Sheffield, Almond Press 1987; Albrecht ALT, The Settlement of the Israelites in Palestine, in idem, Essays on Old Testament History and Religion , Sheffield, Sheffield Academic Press 1989, pp. 133-169).
So all that bloodshed probably never took place (thank God).
At any rate, given that things went as the Bible relates, the practice of herem (OT "holy war", the "devoting to distruction" of the NWT) was not peculiar to the Jews: "holy" wars, "commanded" by tribal gods, and extermination of entire populations in honor of those gods were commonplace in Ancient Near Eastern civilizations (see for instance Sa-Moon KANG, Divine War in the Old Testament and in the Ancient Near East, in Hayim TADMOR, Moshe WEINFELD (eds.), History, Historiography and Interpretation, Jerusalem, Magnes Press 1983, pp. 121-147; Sa-Moon KANG, Divine War in the Old Testament and in the Ancient Near East, BZAW 177, Berlin / New York, de Gruyter 1989; Bustenay ODED, “The Command of the God” as a Reason for Going to War in the Assyrian Royal Inscriptions, in Mordechai COGAN, Israel EPH’AL (eds.), Ah, Assyria .... Studies in Assyrian History and Ancient Near Eastern Historiography Presented to Hayim Tadmor , Scripta Hierosolymitana 33, Jerusalem, Magnes Press 1991, pp. 223-230).