she was to power hungry, her father said no to her request to accompany him on the destruction of the earth nation, her mother visited her, she isnt what you would call stable (she is a demanding perfectionist example: when she was doing her lightning bending training on her boat she got one hair out of place and was furious), she was already on a downward slope from being betrayed by her friends and her failure to capture or kill the avatar, i think that sums it up oh and she was also channeling the crazy side, the sozin side of the family because remember zuko and her are both relaited to avatar roku and sozin, sozin was insane and roku being the avatar was the sane one.