Just say to her... Tell me. are ALL humans good? if she say yes. Mention, Bandits, Rapists, Murderers, Slavers, Corrupted Politicians, etc...
Then, tell me, are all demos bad?. Mention all good demons.
And if she mentions about the wife who was killed... If it was a demon, kill all demons, then if you have a husband and child and a bandit came to your home and killed them... Will you kill all humans?
Good and bad are in everything, its when the bad overwhelms the good side that hell happens.