I understand it was their disobedience but also posit it was because they thought it was the “merciful” way to go...they thought they had killed enough even though God had told them what to do...goes back to the “stiff-necked” crowd that received the 10 Commandments because they insisted that God give them a clear set of rules....they proved we cannot follow even a simple, writ in stone, set of rules.
So you can read minds of people that have been dead for thousands of years ago. Amazing, if true but I think not. In truth you do not know their motive. They could have been tired of killing and therefore gave a half-hearted effort (God is not satisfied with that). But more than likely they saw pretty maidens which inflamed their lust. Or children they wanted. Or slaves. But who knows? They were disobedient and they paid the price.