In considering God’s right to order conquest, we need to first understand God’s prerogative as it pertains to life. Life is a gift from God (see Ecclesiastes 12:7) and everyone in the world belongs to God because they are his creation (see Psalm 24:1). Christians also believe that God is the sustaining cause who keeps us alive (see Colossians 1:17). For these reasons, it is impossible for God to “take” a life in the sense of taking something from a person. Instead we should conceive of God as ceasing to give life.
We can illustrate why this cannot be wrong for God if we think about a friend that you may be giving money to. Perhaps you are giving this friend $5 per day, every day, for weeks on end. If you decide one day to stop giving your friend the money, have you wronged him? Certainly not since it had been a gift all along. He may be surprised by your decision (especially if he has grown accustomed to the income) but he cannot say that you have wronged him. You never owed him the money (it was a gift), so you cannot be accused of wrongdoing because you cease to continually give it. In the same way, God cannot be accused of wrong because he stops graciously giving us life. He never owed us life, and so he has not wronged us if he stops giving it. The man who has had 30 years and no more cannot accuse God because he did not have 31 no matter how many years God may have given to another man. Neither did God owe any more life to any Canaanite who died in Canaan, since the life he had already given them was a gift.
We can think of this another way by considering a loan of something to a friend, say a lawnmower. Your friend may use the lawnmower every week so that he comes to depend on it. If you were to ask for the lawnmower’s return, however, could you be accused of wronging your friend? Certainly not. The lawnmower was yours all along, so you may call it due whenever you like. In the same way, God may call our lives due whenever he chooses without his doing so being immoral. Our lives, such as they are, belong to God and not to us. This is also true of the Canaanites. Their lives belonged to God, and He cannot be accused of wrongdoing because he has called their lives due.