Everyone’s pretty much spot on.
Suppose that, say, China invades Hawaii and conquers it.
Now, suppose that Hawaiians go around collecting taxes on behalf of the Chinese. Suppose they’re supposed to collect ten million dollars. But they don’t just collect ten million… they collect eleven million, and only send ten million to their conquerors, and they keep an extra million or so for their own pockets.
The people of Hawaii would feel really betrayed, right? Because they’d understand that kind of behavior from invading outsiders. But not from people who are their kinsmen. Or who had been American citizens beforehand. And not just betrayed because their kinsmen/fellow citizens had allowed themselves to be employed by the Evil Invaders, but because they were personally benefiting from the situation by taking extra money out of the pockets of their kinsmen/fellow citizens.
So you take a people like the Israelites, who not only live in a common territory, but who are descended from the same tribes, and are kinsmen, and are united by a common religion and a unique culture in a sea of paganism… and all of a sudden, their own people are signing up to work for the pagan conqueror overlords and line their pockets at the expense of their neighbors.