I think that a good start at this problem is to enunciate our ethical theory that underlies our moral judgements. According to the version of divine command ethics which I’ve defended, our moral duties are constituted by the commands of a holy and loving God. Since God doesn’t issue commands to Himself, He has no moral duties to fulfill. He is certainly not subject to the same moral obligations and prohibitions that we are. For example, I have no right to take an innocent life. For me to do so would be murder. But God has no such prohibition. He can give and take life as He chooses. We all recognize this when we accuse some authority who presumes to take life as “playing God.” Human authorities arrogate to themselves rights which belong only to God. God is under no obligation whatsoever to extend my life for another second. If He wanted to strike me dead right now, that’s His prerogative.What that implies is that God has the right to take the lives of the Canaanites when He sees fit. How long they live and when they die is up to Him.