It's not a world with single country by the way, and their neighbours are hostile. That's why they want to keep Rovel around.
Rovel himself also understand he is a war deterrent, so he keep a presence and tolerate the royal family to a certain degree as long as they leave his family alone.
Killing the royal family is short sighted, especially for isekai protagonist who usually wouldn't want to busy themselves with cleaning up the mess and maintain the society.
It will also cause instability, invasion, which brings more casualties.
Bring out the spirits to stop the war ? I'm really sure Origin wouldn't let that happen. The spirits don't want to interfere with the human world like that and it's not their function.
Killing the "evil" then walks out like "there, I defeated the demon, good luck on rebuilding the world. I'm sure you will do fine" is totally irresponsible.