Because she did something to make her envy her.
C would be my best guess but idk.
Once he cut it off, he could use it as a weapon because it's powers still worked, so if someone attacked him he could just whip it out and use it as a weapon to turn his enemies to stone, which he does on few occasions and wins the fights that would either be impossible or much harder for him.
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C. complex characters have flaws and room for growth through the story.