That’s a good question I also have because it the Bible says an eye for an eye, but it also says turn the other cheeck and it says who lives by the sword will die with the sword and love your enemies… so to me it is also unclear if Christians should fight… but in Revelation it talks about a war against the saints, so that implies that Christians will be physicly fighting against the armies of the antichrist… good question, I’d like to know to if someone does have an answer…

The command to kill all in the Promised land (Deuteronomy 20:16-18) was for a specific time and place.

In Genesis 10 it reads:
“15And Canaan was the father of Sidon his firstborn, and of the Hittites, 16the Jebusites, the Amorites, the Girgashites, 17the Hivites, the Arkites, the Sinites, 18the Arvadites, the Zemarites, and the Hamathites (see also Genesis 15:17-20). Later the Canaanite clans were scattered, 19and the borders of Canaan extended from Sidon toward Gerar as far as Gaza, and then toward Sodom, Gomorrah, Admah, and Zeboiim, as far as Lasha.”

It is important to note that Noah cursed Canaan in Genesis 9:25…
“24When Noah awoke from his drunkenness and learned what his youngest son had done to him, 25he said, “Cursed be Canaan! A servant of servants shall he be to his brothers.”

And YHVH said to the people via Moses:
“22But if you will listen carefully to his voice and do everything I say, I will be an enemy to your enemies and a foe to your foes. 23For MY ANGEL will go before you and bring you into the land of the Amorites, Hittites, Perizzites, Canaanites, Hivites, and Jebusites, and I WILL ANNIHILATE THEM. 24You must not bow down to their gods or serve them or follow their practices. Instead, you are to demolish them and smash their sacred stones to pieces.”

In Numbers 13:30-33 it reads:
“30Then Caleb quieted the people before Moses and said, “We must go up and take possession of the land, for we can certainly conquer it!”

31But the men who had gone up with him replied, “We cannot go up against the people, for they are stronger than we are!”

32So they gave the Israelites a bad report about the land that they had spied out: “The land we explored devours its inhabitants, and all the people we saw there are great in stature. 33We even saw the Nephilim there—the descendants of Anak that come from the Nephilim! We seemed like grasshoppers in our own sight, and we must have seemed the same to them!”

So at least for the command to kill, it was limited to a specific time and place. And it appears that the reason they were to be destroyed was that they were descendants of the Nephilim and that the survivors might teach them how to worship their gods and thus turn the people away from YHVH.