What would a compassionate LG king do with a powerful enemy who has been subdued?

Over the last ten years, a powerful lamia sorcerer has eaten untold numbers of people on pilgrimages to a sacred spring. The heroes have knocked him unconscious, antimagic shackled him, and taken him to the king for trial.

The kingdom is recovering from a great war and fighting off the barbarians at the gate. The government is lawful good, so I’m trying to play the king as compassionate as well as just and fair. What would he do with such a powerful, evil person, especially in the current circumstances?