Although Xu Que's words, which even included quotes of some well-known phrases like 'Being alive or dead, I will be a hero who is upright and unyielding', sounded quite sincere, Zixia Fairy didn't believe him at all. When Xu Que tried to escape after ruining her elixir plantation, she knew Xu Que was totally a villain, who was even worse than her exiled apprentice Duan Jiude!
A bad guy like Xu Que would definitely be a public enemy in her age.
Also, Zixia Fairy was curious how a bad guy like Xu Que could stay alive till now. She thought the way of the world had been changed.
Therefore, she made a decision that the first thing she would do after going out was to kill Xu Que to prevent him from doing evil to this world!
Of course, if she went out now, she would find Xu Que had already become the public enemy of this world.