They weren't so innocent
Over and over we hear, "I couldn't worship a God that would command Israel to wipe out other nations. Men, women and children were slaughtered at the behest of a God that is supposed to be a god of love." People with these types of arguments are assuming a huge assumption here. They are assuming that these nations were full of innocent people shopping at the mall and eating a peaceful dinner when the big meany Israelites egged on by a bloody ferocious god invaded and killed them all. Nothing could be further from the truth.
One of the reasons the Law was given at Mt. Sinai was that people in general were doing all the bad things forbidden to Israel. The wickedness of the nations was so bad that it hearkened back to the time just before the flood when every thought was only evil continually. A partial list could include many of the things the nations do today such as abortion and homosexuality, but there were also some specific terrible practices. They burned their sons and daughters in the fire to their gods. They used divination, told fortunes and interpreted omens like modern witches or astrologers. They were sorcerers and charmers and mediums. Necromancers or inquiring of the dead (as with séances) were other staples of their religious practices. All of these practices are associated with demons, goat demons (Leviticus 17:7) and the deceiver.
“When you come into the land that the Lord your God is giving you, you shall not learn to follow the abominable practices of those nations. There shall not be found among you anyone who burns his son or his daughter as an offering, anyone who practices divination or tells fortunes or interprets omens, or a sorcerer or a charmer or a medium or a necromancer or one who inquires of the dead, for whoever does these things is an abomination to the Lord. And because of these abominations the Lord your God is driving them out before you. You shall be blameless before the Lord your God, for these nations, which you are about to dispossess, listen to fortune-tellers and to diviners. But as for you, the Lord your God has not allowed you to do this. (Deuteronomy 18:9–14, ESV)
They Took Depraved Behavior to Extremes
The nations were indulging in all sorts of perverted practices such as men laying with men as with a woman, or a woman laying with a woman as with a man. They had sexually immoral relations of all kinds, even with animals. They had so degraded themselves that there was nothing left but to extinguish their unclean lives with the sword. Things were so bad the land itself was vomiting them out.
“Do not make yourselves unclean by any of these things, for by all these the nations I am driving out before you have become unclean, and the land became unclean, so that I punished its iniquity, and the land vomited out its inhabitants. But you shall keep my statutes and my rules and do none of these abominations, either the native or the stranger who sojourns among you (for the people of the land, who were before you, did all of these abominations, so that the land became unclean), lest the land vomit you out when you make it unclean, as it vomited out the nation that was before you. For everyone who does any of these abominations, the persons who do them shall be cut off from among their people. So keep my charge never to practice any of these abominable customs that were practiced before you, and never to make yourselves unclean by them: I am the Lord your God.” (Leviticus 18:24–30, ESV)
“You shall therefore keep all my statutes and all my rules and do them, that the land where I am bringing you to live may not vomit you out. And you shall not walk in the customs of the nation that I am driving out before you, for they did all these things, and therefore I detested them. (Leviticus 20:22–23, ESV)
It Was A God Thing
It wasn't because Israel was so wonderful that they were used as God's cleansing agent. Israel had it's own problems with obedience to God. It was because the nations, especially seven of them, were so desperately removed from anything resembling goodness or right and healthy living.
“Do not say in your heart, after the Lord your God has thrust them out before you, ‘It is because of my righteousness that the Lord has brought me in to possess this land,’ whereas it is because of the wickedness of these nations that the Lord is driving them out before you. Not because of your righteousness or the uprightness of your heart are you going in to possess their land, but because of the wickedness of these nations the Lord your God is driving them out from before you, and that he may confirm the word that the Lord swore to your fathers, to Abraham, to Isaac, and to Jacob. “Know, therefore, that the Lord your God is not giving you this good land to possess because of your righteousness, for you are a stubborn people. Remember and do not forget how you provoked the Lord your God to wrath in the wilderness. From the day you came out of the land of Egypt until you came to this place, you have been rebellious against the Lord. (Deuteronomy 9:4–7, ESV)
The Law was given to set standards of good, right, healthy and holy behavior. It was to remind people everywhere that doing your own depraved thing was going to send you straight to hell, do not pass go do not collect two hundred dollars. It is an objective standard that when followed in love will bring you closer to God instead of further away in depravity. His people are not even to inquire about other gods or methods of worship. We are to stay focused on His ways alone.
“When the Lord your God cuts off before you the nations whom you go in to dispossess, and you dispossess them and dwell in their land, take care that you be not ensnared to follow them, after they have been destroyed before you, and that you do not inquire about their gods, saying, ‘How did these nations serve their gods?—that I also may do the same.’ You shall not worship the Lord your God in that way, for every abominable thing that the Lord hates they have done for their gods, for they even burn their sons and their daughters in the fire to their gods. “Everything that I command you, you shall be careful to do. You shall not add to it or take from it. (Deuteronomy 12:29–32, ESV)
Depravity Starts with Small Compromises
Complete and utter depravity is not a place that is arrived at overnight. It is a destination that starts with small steps and with people thinking that little things like Christmas trees and astrology are harmless. Just because we aren't zapped with a lightning bolt immediately when we play with a Ouija board does not mean that it isn't the first of many small steps on the way to destruction.
“If there is found among you, within any of your towns that the Lord your God is giving you, a man or woman who does what is evil in the sight of the Lord your God, in transgressing his covenant, and has gone and served other gods and worshiped them, or the sun or the moon or any of the host of heaven, which I have forbidden, and it is told you and you hear of it, then you shall inquire diligently, and if it is true and certain that such an abomination has been done in Israel, then you shall bring out to your gates that man or woman who has done this evil thing, and you shall stone that man or woman to death with stones. On the evidence of two witnesses or of three witnesses the one who is to die shall be put to death; a person shall not be put to death on the evidence of one witness. (Deuteronomy 17:2–6, ESV)
So Israel was commanded to wipe certain nations off the map because there was nothing redeeming about them at all. God did the world a favor by the destruction of certain nations instead of destroying the whole world as He did in the flood. God slowed the depravity by extinguishing some specific nations and setting up Israel with the Law as an example of what should happen when we choose His ways of life.
But in the cities of these peoples that the Lord your God is giving you for an inheritance, you shall save alive nothing that breathes, but you shall devote them to complete destruction, the Hittites and the Amorites, the Canaanites and the Perizzites, the Hivites and the Jebusites, as the Lord your God has commanded, that they may not teach you to do according to all their abominable practices that they have done for their gods, and so you sin against the Lord your God. (Deuteronomy 20:16–18, ESV)
You Thought the Previous Destruction was Bad?
Sadly, although God slowed down the depravity, modern nations are on the way to exceeding the former depravity of those other nations by orders of magnitude. Baby killing is still with us though they might not be thrown into a literal fire as before. Witchcraft (using drugs) has been given new twists with scientific research into things better left alone. Genetic scientists are trying to cross breed species that should never be. Demonic activity is rising even if we don't recognize it as such but you can see it in movies, music and science all over. So-called Christian entertainment has part-goat people who are supposed to be good. Sexual immorality is hitting new heights of acceptance (or depths as it were) as "alternate lifestyles" even in the Church. This time He is not going to slow it down or just set aside a witness nation such as Israel. Soon He is going to permanently end the depravity and consign it to a dark hot place where it won't bother anyone anymore. The land will vomit out in violent upheaval all who practice such things.