They cut out something pretty huge from the LN here. He hears the voices of the 5 soldiers he killed, and pulls out the spear and asks forgiveness for not saving them too.
Then the bones shape into a sword. The dead soldiers are reaffirming his humanity, and he used the sword to cut the chains
Then the bones shape into a sword. The dead soldiers are reaffirming his humanity, and he used the sword to cut the chains