Does evil exist?

Yes, within the dualistic framework. Evil is manifestation of ego (as is everything here) but taken to an extreme as evaluated by consensus. In other words, evil is a relative truth; you can't find anything that every single person who has ever existed will agree is evil. It is pretty easy, though, to get the people who are in your close circle of proximity to agree with your view of what evil is.

Outside the dualistic experience, evil clearly cannot exist. Remember, in non-dualism, only God Is. Nothing can be Real that God didn't create, since God and Reality are synonymous. The idea of something opposing God is ridiculous and laughable. (This is why, incidentally, the symbolism of light energy is very useful. Grammatically, the opposite of light is darkness, but they are not really opposing forces in the physical world. Light is real energy, and darkness is just a word we made up to mean 'light being blocked.' Same with heat and cold.)

Sometimes students of non-dualism will take their previous experience with dualistic thought systems and try to reconcile the two thought systems by saying that ego is the devil. In a sense, this is correct. Ego does attempt to be our 'god' during our earth experience, ego is worshipped and served, and the vast majority of prayers offered by humans are actually prayers to ego, not to God. However, the best ego can do to usurp God is to temporarily distract us from Reality. It's a crazy idea, but the fact that you believe you are you and I believe I am me indicates that we bought it. In this sense, all humans are equally 'evil' because we all believed we were egos.