Being ruthless to everyone would just make more enemies than you want anyway. He needs Youmu's cooperation for the future, so angering him now would not have been the best move. Plus now he has one of Clayman's more important subordinates in his debt, which I'm sure has plenty of value for whatever sort of retaliation he may intend later on. She might be a tool, but she's bound to have some intel. Her magic expertise would be useful as well.
Genuine kindness is definitely a part of it but it's also tactically sound. I'm sure he's learned to not blindly treat people well without sound reason after what just happened in his kingdom...