The Israelites aren't exactly walking into unoccupied territory.
The Anakim, a proud and powerful people, control the land. But the Israelites shouldn't fear the Anakim's strength and height because God is awesome. To be specific, he's a consuming fire who will stop these giants.
Moses tells the Israelites: don't think God is giving you the land because you're righteous. It's actually because the people of the land are evil, and because Abraham, Isaac, and Jacob were promised the land. Oh, and you almost didn't get it because you're so stubborn. Well, that's nice.
Notice how God judges all people—he's in charge of everyone. So why don't the commandments (like the one not to kill) apply to everyone?
This covenant is like a contract. Because God and the people have a deal, the people live by certain rules; God has no deal with the enemies of the Israelites, and so the Israelites are free to plunder and pillage their opponents. What do you think of this logic?
If you don't like being reminded of your faults, be glad you weren't an Israelite. Moses constantly reminds them of their rebellious and stubborn nature. Their worst offense occurred when Aaron created a golden calf (Exodus 32). When Moses stayed too long on the mountain, Aaron created a golden calf in response to the people's demands for a god. God wanted to destroy them and choose another people, but Moses pleaded with God on behalf of the people, and God changed his mind. Thanks, Mo.
Moses came down from the mountain with the tablets of law and smashed them. That's right: he literally broke the law.
Moses continued to fear for the people because of the Lord's anger, but he again convinced the Lord not to destroy the Israelites, and finally burned the golden calf. Whew, that was a close call.
Moses essentially says, "Hey Israelites, let me remind you of other times that you did bad stuff, especially when you didn't trust God by refusing to take the land." There's nothing like a pep talk, eh?
Moses reminds the people how he continually interceded on their behalf. You think he wants them to feel guilty because he won't be entering the Promised Land with them?
Think about it in terms of when the text was written. Deuteronomy talks a lot about how bad the Israelites are in terms of perfectly upholding this contract. It sounds to us like pieces of Deuteronomy were an attempt to explain a tragic event in Israelite history. Just something to chew on.