‘ And she said to the men, “I know that YHWH has given you the land and that your terror is fallen on us and that all the inhabitants of the land melt away before you.” '
Here is a clue to her behaviour. News and rumour had spread widely while Israel were capturing the land Beyond Jordan, brought probably by those who fled from them. The news was about this terrible nation with their terrible God, YHWH, Who seemed invincible, a nation who claimed He had given them the land of Canaan and that they were coming to take it.
“Your terror is fallen on us -- the inhabitants of the land melt away.” This was as Yahweh had promised Israel (Exodus 15:15-16; Deuteronomy 2:25; Deuteronomy 11:25). The gist of her conversation is translated in words reminiscent of these promises. She would, of course, be speaking in a Canaanite dialect, not in pure Hebrew.
LXX omits ‘all the inhabitants of the land melt away before you.' Perhaps it was not in their copy of the Hebrew text (compare Joshua 2:24 where it is in LXX). Or perhaps they were abbreviating the text. LXX in Joshua is based on a shortened text and the translators were ready to be quite free with it.
“For we have heard how YHWH dried up the waters of the Sea of Reeds before you, when you came out of Egypt.”
Compare Joshua 9:9-10 also Exodus 14:21. The story of what happened at the Sea of Reeds had become famous, passed on by travellers and storytellers from mouth to mouth, no doubt improving as it went. Most of Canaan would have delighted in the discomfiture of the Egyptians, and the story would have brightened many a weary night until they suddenly learned that the same people were now threatening their own borders.
“And what you did to the two kings of the Amorites who were in Beyond Jordan, to Sihon and Og, whom you utterly destroyed (‘devoted').”
See Numbers 21:21-35; Deuteronomy 2:26-36 especially Deuteronomy 2:34; Deuteronomy 3:1-12. The practise of ‘devoting' conquered people to a god and destroying them was known elsewhere and was common. In the Moabite Stone we read of Mesha devoting the city of Nebo to his god(s) Ashtar-Chemosh and slaughtering all its inhabitants. This practise was, in the case of Israel, reinforced by the fact that Israel must not live among the Canaanites and Amorites, but must destroy them or drive them out because of their debased religion, lest they themselves become corrupted by it. The Canaanite religion was a religion obsessed with perverted sex, distorted but physically attractive. But the news of the intention of the Israelites was sufficient to chill the heart of those waiting for an invasion to come.
We may sometimes question why they behaved so harshly, but we need to recognise the harshness of the times, and the necessities that were laid on them (as well as God's right to bring His judgment in any way that He decided was right). Everyone in Canaan (and elsewhere) accepted that they themselves had a right to possess other people's land and drive out the inhabitants. That was not open to question. The only thing that prevented it was their weakness or strength at any particular time (what happened in Judges when there were strong kings over different nations brings this out). The Amorites mentioned here had refused Israel safe passage along the King's Highway. In other words their threat had been that if they did not go back, or if they tried to take the road though their land, they would slaughter them all, men, women and children. Israel had been left with no alternative but to reply as they did, for the alternative was to leave alive an enemy who at any moment could rear up against them, having obtained reinforcements, and Israel had no cities in which to guard their women and children. In such circumstances the only ‘good' Amorite was a dead one. As for the Canaanites in the future. They would on the whole resist Israelite occupation of the land tooth and nail. They were not peace loving nations suddenly attacked by a warlike Israel. Israel were in constant danger of attack from them. Even though much of the land that they initially occupied was uninhabited no one would cede it to them. They had to fight every inch of the way. But added to that were the evil practises which were a part of the Canaanite way of life. They were probably riddled with sexually transmitted diseases due to their sexual perversions, and mingling with them would have destroyed Israel both spiritually (as indeed it did in the end) and physically. The only path really open to them, as YHWH had made clear, was either to drive them out or slaughter them.