Kagha isn't justified in what she is doing, although she feels the opposite is true. That is kind of the entire point of her character. She is a druid, but with extremist and xenophobic leanings.
How is she not justified? You are projecting your views on her. According to her own religion she is well within her rights to do exactly what she does, and if the ritual works that pretty much means that Sylvanus approves. Again, xenophobia against extraplanar outsiders is not something I would describe as "wrong" according to the value-system of a Sylvanus-worshipping druid. Sylvanus doesn't care about your morality as long as you get the job done.