I actually think it is better for her to stay alive. If we think it in simpler way, it is easier for Ark to just massacre those who might assist the prince and hero one day. But by killing them, it might open the eyes of both the spoiled prince and hero. It is better to keep both spoiled to the rotten so they can't get stronger.
Mist is so cool... Leone might fall in love to him if he is the main character.
I think more than that, by letting them live for now, he is able to slowly and thoroughly destroy their backing and every supporter they could possibly have so when there is no chance of revenge against him or allies he can eliminate them without repercussions. Surely the princess will be the one for the throne instead of the prince and Ark will likely be king unless she renounces it to fall for Mist instead.
Do anyone have the proper raws for first 10 chapters? The previous idiotic and retarded RESET SCANS censored and removed several pages out of their releases because those were not ad friendly.