My theory is that her former self is still in there, or rather, she is that same person, just warped- I think that the adventurers she rescued in the flashback we saw came back and killed off all of the fairies to grind them up for pixie dust or something similarly fucked, which caused her to snap (understandable) and kill the ones responsible. I think then as she was killing them, she found she really, really enjoyed it, beyond just getting revenge, and so to try to isolate herself (somewhat, she'll kill the MC who was captured+others Kendra will capture, so it's not 100% obviously) from innocents, she moved to the most twisted part of the continent, Hornsperia.