So God didn’t want the Israelites to intermarry with the people of that region. But if God were actually merciful and loving, was there no other way to prevent the Israelites from intermarrying the indigenous people of the promised lands, or was slaughter the only possible option?
An omniscient God would have known that this problem would arise, and he could have planned for it at the time when he created the sun, the moon, the planets, the stars, the galaxies, and the earth itself. An omnipotent God could have created a force field around the entire region to prevent anyone else from entering. Then, when the Israelites arrived, he could have disabled the force field to let the Israelites enter a region that would have been fertile, lush, and unpopulated.
Or alternatively an omnipotent God could have left the promised lands at the bottom of the Mediterranean sea. Then when the Israelites arrived he could have lifted up the promised lands from the sea floor and the Israelites could have entered a region that would have been pristine, new, and uninhabited.