I'm exhausted and have, over the past few days, gotten into too many debates for my own good but I'll give a small answer. If you want to have a dialogue about this just send me private messages, I prefer that.

First, like most of the ideas and questions that we face you have to work from a Christian framework and a Christian conception of God. The view of God then works its way back into the definition of life and the situation that arose in the OT.

Second, we'll look at the narrative of the Old Testament and what was happening exactly in the story of Israel.

Third, we'll allow for one to disregard that completely and work from a perspective whereby we assume Israel was working from the will of God in the first place and talk about why it was still not terrible.

I'm not going to create a dichotomy between the OT and the NT God as I believe there isn't one.

(Note: I'm not going to provide evidence of God, please allow me to assume that God exists as the Judeo-Christian tradition understands him so that I can simply answer this to show how the issue is not a problematic one)

So the idea of God: Well, the Bible says that God is a loving, just, omniscient and omnipotent deity. He was before everything and from him everything traces its origin. Because he is the creator of it all he is also the rightful owner for all that exists. This is important because we tend to think that life itself is owed to us, whether it be long life, comfort or wealth. This perplexes me because it implies that I have a way to judge what is due me. So since God is the creator of all things the very fact that he wishes to take a life (or return a life to him since death is not the end) is not something that is wrong. Murder, rape, and all of those crimes against others are so heinous because not only do they harm another individual but they assume ownership over a life that is not assigned to us. So, from this we can gain a view that God simply "taking a life" is not wrong by any objective view of justice. Furthermore, our view of our own purpose tends to skew away from what I view God's as. For instance, you mention razing nations to the ground as if the very act of destroying a construct of man was a bad thing. Ignoring the loss of life would God instigating the destruction of a building against our will be a bad thing? I don't think so. What about a government? Also, I would argue not. So without going into any specifics we may come to a conclusion that the very question itself is a curious one indeed.

But let's disregard that for a second and say that I've simply spoke gibberish. The second explanation (which only further demonstrates that God is not cruel) is the narrative of the old testament. I had a post on this in this very subreddit a while back so I'm going to copy and paste it. Basically, we tend to view what happened in the Old Testament as the specificities of God's plan when in fact the the specific instances are diverted courses but the plan as a whole goes directly where God wants. Let me explain.

Why did God create man? Why not stick with angels? Why make anything at all? He obviously doesn't need us. We don't give him anything he doesn't already possess and he was in existence long before we poked our heads into this universe.

The simple answer is that God created us because of an aspect of his character. God is omniscient, omnipresent, omnipotent but he is also loving. That is the key. He creates angels and man so that they can enjoy an aspect of his nature or many aspects to his nature. Angels can see his power, they see the extent of his divine will but they cannot see a certain perspective on their own. They cannot see his love and grace.

So God creates man with that intent. Man will be able to enjoy him in a way that angels could not.

God creates man so that he can choose to follow God and thereby see God's benevolence and love. God creates man with one divine edict, and this is key, that man will choose God over all else and that God will walk amongst man in unadulterated glory.

That is the foundation for everything we see in the scriptures. It is a divine edict that man can turn away as much as a raindrop can seek to divert the tide.

So God creates man with this purpose and man turns away. It's hard to imagine rectifying that situation. It would seem that this divine edict has been turned to a trite piece of failure. But that is when you get the message loud and clear: nothing we can do can turn God from the ultimate purpose.

Man is cast out from the Garden of Eden and wanders. Eventually you come to an old man in the middle of the desert and his baron wife. God tells them that his plan is only beginning to be shown and he chooses them to begin his nation. They laugh, how can God give them children, he must be joking. And God turns that on his head by giving them a son and telling them to name him Isaac or "He laughs/will laugh" for God's plan cannot be undone and his will cannot be avoided. He tells Abraham that he will reach the world through Abraham's children and that through him all will be blessed. So from the patriarchs God founds a nation despite all of the patriarchs doing their own stupid things trying to either help God out (like having a son through another woman) or trying to do things that would actively end God's will (like killing Joseph) but all for naught because God's plan is greater than the expanses of our vision.

This is where the story really comes together.

Israel is in captivity in Egypt. How will God do anything with them when they are slaves? God picks the most unlikely person to reach them. A son of Israel that has grown in the house of Pharaoh, been educated by his men, but cannot speak with any elocution. God uses this man to free Israel.

Now there is something that could have happened. Israel could have followed God and an alternate history would have transpired. Israel would go through the desert straight to the land promised to them. The peoples of the land knew that God was with them and they would willingly allow Israel to pass and set up a temple for the world. Israel was a priest nation, what should have happened was that they should have set up a temple and behaved as priests, ushering others to the one true God. People would have known the power of God was behind them and they would follow those who worshiped the one and only Lord. Christ would have entered the world but not hidden as a child, but exalted as a savior. He would have to die but would be offered by the high priest as a replacement sacrifice for the lamb that was usually used. It would be intentional but not willful. Christ would die, be buried and rise again three days later and immediately enter Israel's temple and usher in the new heaven and new earth.

But that didn't happen.

Israel was the worst possible people to choose but God was showing something through them. He was showing that his edict would come to be despite the failures of man. Israel didn't want to leave Egypt once they got to the desert, but God freed them anyway. Israel wanted to replace God with idols but God protected them anyway. Israel feared the people's of the promised land but God gave them the land anyway. Israel became a wayward people but God demonstrated his power to the surrounding nations regardless. Israel refused to worship God but God ensured the safety of the faithful. Christ came to the world as God intended from the moment sin entered into the picture but not as a savior. He was ignored, ridiculed, beaten, and mocked. But it didn't matter, because God had made his edict long ago. Israel sacrificed Christ but not in the manner they should have.

Israel failed to reach the world but that didn't matter, because God had told man that he would walk amongst them in purity. So Christ does not enter the temple upon coming back to life. Instead he forms a people who's shear love for him would spread across the world like a wildfire and ignite the hearts of men and women from every generation and every people. Once every people had been reached God would enter in the new heaven and new earth and man, purified by God and saved by grace, would walk with God in purity and righteousness and admire him for his omniscience, omnipotence and love.

So what we see when we look back to this is that the intention (initial intention) is not to wipe out these nations. That is a consequence of Israel not following God's instruction. Had Israel been completely faithful they would have walked into the Promise Land and the other nations would have seen the glory of God marching with them and followed. But because of Israel's lack of faith the other nations fell. If the priest is corrupt do you expect the followers to be saints?

The final point, and this is in regards to the specific instances. It would be preferable to address specific instances separately as doing one sweeping answer will lose a lot of information and be more prone to showing holes because I'm not addressing things directly. Many of the nations were actually evil ones not simply in the way of Israel. Child sacrifices and other such corruptions permeated many of the cultures and these things easily corrupted such a weak nation as Israel so they had to be removed. It is not enough to scold cancer, sometimes you have to cut it out. Furthermore, many of the nations new, very clearly, that God was directly behind Israel and the act of getting in their way was an act of defiance against God. Finally, as many of the nations were subjugated before Israel so too was Israel subjugated by nations as per the will of God. We tend to view it as one sided but really God uses nations to punish other nations throughout history.

That's it...verbal vomit done...I'm going to bed.

Edit: God, I have to edit something because I left out a huge detail in the second point. That's incoming.