I assume you refer to her refusal of peace? I think that was a monumental success for the Light. If Rand had won that one, he would have felt justified in his terrible handling of the world around him. Tuon's refusal to back down in the face of a man who could kill her within a moment won her even more respect from me.

I think the important thing to note here is just as you sometimes have people on your side whom you wish were on the other (eg the Whitecloaks), you sometimes have people on the other side who demonstrate traits you wish were on your side. Tuon might have been bad for the Light, but as an individual I like her and I think she could be the most competent Lightsider after Rand if she joins forces with him.