It wasn't necessarily kindness on her part. She's just been revealed as a witch who cast a curse upon her own country after living for years as a recluse, both before and after her parents were killed under unfortunate circumstances. She isn't exactly coming to the throne with lots of goodwill on her side, whether it's from her own people or other nations. Hans may be the last of the line, but he's still a prince, and her ordering his death could seriously piss off more than just a few nations, especially since she's already tossing the Duke out as well. Deporting him forces the ball into his family's court so they're the ones who have to make the ugly decision: punish him, which fulfills a sense of justice and keeps her and her magic happy, or not punish him and suffer the consequences. Either way he's out of her hair, for now.
Or she may not want to start her reign with death. Whether it was a diplomatic move or not, she doesn't really need to care for him to make it, considering she was pleased when Anna punches him into the water.