Rereading this arc, it's interesting to note that Granite's brief explanation of why she hates ice magic fits with an earlier comment about how she prefers things to be absolute; when she kills an enemy, she wants the satisfaction of seeing them cease to exist.
Also, I'm wondering what she'll do after this. It seems obvious she hasn't resigned herself to always using ice magic, but there's no reason to believe that she'll be any more capable of using fire magic than before.