Is a genocide ever considered necessary?

Morally speaking you are getting todays morals mixed up with God's wisdom, justice, and authority. You can ask him yourself on judgment day. I am sure he will answer you. Perhaps the answers lay some where in the vacinity of the canaanite's refused to believe in God and were so interested in their own high ideas and idols that they forgot to pay homage to the one true God so he ped judgment on them ealier than on the rest of us.