Where did it indicate she did learn her lesson? She cried when her father left. Did she express any regret for what she'd done? That 600,000 people died, even momentarily, because of her actions? She didn't understand, and that's very different.

Calling Ben "unforgiving"--he's justified there. He and 599,999 other people died because Charmcaster thought their lives didn't mean as much as her father. They would all have stayed dead had Spellbinder not been horrified at what happened and decided to go back on the deal. If Ben can't forgive Charmcaster for that, then I really can't blame him.