:: God had mercy on Sodom for the sake of 10 righteous men. ::
I would think destroying an entire region through raining-down fire and brimstone is the exact opposite of mercy. Lot’s family (less than 10) were provided a chance to leave but even his wife proved her disloyalty.
But God promised that He would show mercy to Sodom for the sake of ten righteous men...there just weren’t ten in Sodom.