One account that bugs me is that of Achan, as reported in Joshua ch.7. True, the guy did wrong. But Joshua, in the attempt to single out the perp, says "My son, render, please, glory to Jehovah the God of Israel and make cofession to him, and tell me, please, What have you done? Do not hide it from me" to Achan, and when he fesses up, and if one looks at verses 20 and 21 he actually sounds repentant, or at least sorry, and yet he AND his whole family plus everything the guy owned was destroyed.
The only way I can somehow reconcile this account with a merciful God is that maybe Achan was not really sorry (or he was just sorry he got caught) and that God was able to see through it to his real heart motive. But if that is not the case, then it seems to me that a guy who confessed ought to have received more consideration.