They would have harmed all of the animals and insects living within the forest,
and they would be brutally breaking their promise from before by making the deaths of the people who sacrificed their lives for the trees and animals look meaningless, and the king would be breaking his promise to the people by falsely telling them fake information
You didn't give context but, I believe it's most likely D. He knows that his school is dangerous and that his fellow enslaved people are brave to attend