The non-duality philosophy idea seems to be that polarity is an illusion, because in reality everything is connected and whole. Even when things seem separated they are still connected and depend on each other to define and create themselves through their connection. So if evil depends on good to exist than does it really exist if in the absence of good is evil?
From Satischandra Chatterjee, Dhirendramohan Datta. Introduction to Indian Philosophy. University of Calcutta, 1948 :
The critics of Advaita Vedanta have often urged that if Brahman be the only reality and all distinctions false, the distinction between right and wrong also would be false. This objection is due to the confusion of the lower and higher standpoint. For one who has not yet attained liberation, any action which directly or indirectly leads him towards the realization of his unity with Brahman, is good and that which bampers such realization, directly or indirectly, is bad. One who has attained perfect knowledge and liberation would look back upon these moral distinctions as being relative to the lower standpoint and, therefore, not absolutely valid.
this book also refer to Radhakrishnan, Ind. Phil., Vol II, pp. 612-34, and speeches of Vivekananda quoted by James in Pragmatism, pp.152
Of course there is such a thing as evil. It is the moral evaluation most people attach to things like eating babies, murdering kittens, torturing for amusement and other horrid acts. The thing is, while the OP, myself and possibly many others regard, say, Adolph Hitler to be an evil man, there is unfortunately no such thing.
Psychologically realistic bad guys see themselves as heros. I do not see the bad guys as heroes, I see them as bad guys (and I am sure most agree). The terrorists behind the Madrid train bombings no doubt thought of themselves as martyrs of Islam, dying to protect all that is right and good.
But we don't see them that way. I must again emphasize that thinking you are a hero does not make you one.
Evil is a thing inside our brains, and every human has the notion of it, and the hitch is that you never (granted you are mentally healthy) think it applies to you.