Why the slaughter?

“The Lord also gave that city and its king into Israel’s hand. The city and everyone in it Joshua put to the sword. He left no survivors there. And he did to its king as he had done to the king of Jericho." Joshua 10:30
Many people come across accounts like this, particularly in the Old Testament and with regard to the commands to completely annihilate all of the Canaanites, and struggle to understand why God would condone, let alone command, such a thing. Many see God as a monster because of this. But God is not a monster, and He has good reasons for commanding that all the people be destroyed: “Do not intermarry with them. Do not give your daughters to their sons or take their daughters for your sons, for they will turn your sons away from following Me to serve other gods, and the Lord’s anger will burn against you and will quickly destroy you” (Deut. 7:3-4).
God had chosen the nation Israel to be His special instrument in the world, to show the world how a nation that obeyed God could be blessed. He knew that the Israelites, like any nation, were easily swayed into idolatry. The idolatry that the Canaanites were involved in was of the worst kind – obscene sexual rituals, child sacrifice, the works. They were on a path to destruction anyway, like a rabid dog, beyond cure. There is also the issue of the gene pool being contaminated by the Nephilim (“and also after that”, Gen. 6:4, see Num. 13:33).
God also calls for a cutting off in our own lives of anything, inherently sinful or not, that would draw us away from Him. It is not to spoil our fun, but to protect us from contamination. We need to see these things from God’s perspective.