Lia will just put up walls that could have already come down, and even, at first anyway, when Kai tries to reason with her she just turns away. 

And Z sees it and just gently reminds Kai that the problem with being pure is accepting how dark you can become.and that she just needs some time to cope with what happened. Being a creature of light himself.

Just like if Kai ever.had.to produce massive amounts of light magic, it could be equally as traumatic as going from light to dark.