WT and other groups who believe the bible is infallible say:
1. Those nations (like canaanites) deserved it, as they did very bad things.
2. God gave life, God can take lives. Who are we to judge him?
3. God will have some reasons for his actions that we don't have.
Some of my friends and family say that Jehovah will ressurect some that he killed. They say it would have been better for the canaanite babies to be killed by the sword than to be left on their own to die.
I can't accept any of these explanations.