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
The theme of the poem is death and the inevitability of it. The author is trying to make themselves one with nature and its ugly surroundings that will over power him.