In the KJV the word for taking another person's life in battle was 'slay', while the word for taking another person's life in order to get what he had (such as a bandit) was 'kill'.
As for what happened to the Canaanites, that was how wars were waged in that era. The troops invaded the city, wiped out everyone who lived there, and then left. Archeologists only a few years ago found a city buried under the sand. When they excavated it, they realized that there were bodies still lying in its streets. An invading army had attacked the city, killed its inhabitants, and then moved on, leaving it for the desert to bury.
Moses, the leader and commander-in-chief of the Hebrews, would have known how to fight this kind of war. According to Josephus' Jewish Antiquities Moses was a general in pharaoh's army, and had himself led that army into battle in Ethiopia, crushing an uprising there. Having been adopted by pharaoh's daughter, and if the suspicions of many of us are correct also a pharaoh's wife and Queen of Egypt herself, Moses would have learned egyptian military tactics and weaponry. He would also have been indoctrinated in their mindset, namely, if you have to attack a city, make an example of it for all other cities to fear. Egypt didn't become the most powerful ancient kingdom by being nice guys.
It would have been this type of warfare that Moses would have taught the ragtag mob that was the Hebrew nation once he got them out of Egypt. We look on it with a sense of horror, but think nothing of dropping bombs from 25,000 feet that we know will take the lives of 10's of thousands of people. It is said that the attack on Tokyo in 1945 that created a massive firestorm, killing well over 100,000 people (and according to a survivor of it whose son I personally knew killed closer to 500,000), took more lives by itself than were taken in all of the wars fought before the time of Christ.
In the 'for what it's worth' category, there are those of us who believe that the person identified as 'pharaoh's daughter' was Hatshepsut, daughter of a pharaoh, chief wife of a pharaoh, Queen of Egypt, and eventually the only female to successfully take the title of 'pharaoh' herself. She would have had the clout to save Moses. She is also the person whom we believe named him (Moses' is the hebraicized version of 'Thutmose', the name of Hatshepsut's father, husband, and stepson).
