Religion says that evil is a distortion of good and is not a substance or anything like that. It is defined as a lack of a good that needs to be there and is not. In moral terms, somebody is causing the lack. Supposedly, you turn from the good you should aim for, for a lesser one and the side-effect of that is harm. That is the teaching but it is one of those things you cannot convince anybody of. You cannot prove its moral significance. Imagine he says, "The evil I do works within good. It depends on it. So it will not last and give rise to good. Therefore I cannot see how I should care a lot if I do it." You cannot command him to throw that thinking out. You cannot hide behind a God to do it. There is also the problem that the doctrine isn't easily understood or very relevant in daily life. People are not comfortable with the idea of mass murder or genocide being a mere lack. It is not going to have an impact.
We are about to explore what happens if some force can feed information about evil to you as if it were good. We have learned that we already have perception problems there. Those problems provide evidence suggesting that such a force may be at work.
Let us leave that behind and take a thought experiment.
There is a useful lesson in horror films where some monstrous creature is perceived as your beloved mother. The paranormal, divine or supernatural alters your feelings and your senses so that the wrong information goes to your very core. It could even be an alien super-science. Whatever, we may call it a higher interference.
Something nice is happening to you. Some greater power, is making your mind or some sense, read it as an evil attack that gets in and hurts you from the inside. What if evil is indeed all in the mind in that way?
Something evil is happening to you. Some higher interference is making your mind, some sense, read it as good and nice.
What if good is all in the mind?
What if it is not good but not evil either? Something non-moral or neutral?
If it is something neutral that means it has both bad and good sides or neither if you like. The higher interference can deceive you by turning off your detectors of the good so you only see the evil and you condemn it.
The point is that what matters is how you do not know. Claiming to know would be a lie. It would be strange if you have to lie in order to validate your awareness of good. Goodness that is propped up by pretending to know more than you do is going to be fragile. Real good is strong.
You are touching good and it is perceived like something evil. You are touching evil and it is perceived like something good.
You are touching evil and it feels like something non-evil or even good.
You are touching evil and it feels like something non-evil or even good.
You can touch a pink flying pig when drunk and that does not mean it is there. A pig is bigger than you in a sense. It is outside you and you cannot do anything about it. It is there and it is physical. It does not care what you want or think. You have to deal with it being real whether you like it or not. If you can be mistaken about something like the pig you can be mistaken about what your intention is - good, bad or neutral. If the higher interference can show you a turnip and everything tells you that it is a pink pig, he can do the same for your intention. You detect it as good one and think you implement it as such. The truth is it is a bad one.
RESULTS
Nobody can logically prove in any particular case that
1 their good, evil or neutral experience was accurate
2 they had a good, evil or neutral intention
3 They cannot answer, "What if good, evil and neutral are never ever absolute external realities, but entirely mental experiences? Or mostly never?"
If they have no reason to affirm any higher power or higher interference they have nothing to worry about.
If they do, they take a step closer to doubting everything even if they are not doubting yet.
They lose any right to condemn the person who says they are doubting. They have to affirm and support that person even if they disagree. If we want moral clarity to avoid chaos, it is not going to help when persons can legitimately doubt.
FINALLY
Being epistemologically skeptical is forced on us. It questions whether we can ever truly know good or evil, given the potential for mental misinterpretation or higher interference.
Being metaphysically agnostic is forced on us. It asks whether good and evil exist outside the mind or are entirely mental constructs.
If anything is forced on us, we wonder why it has to be forced. This gives rise to doubt.
If you face God or Satan himself, we are forced to guess which is which. Satan would wear God's face in addition to that. That makes the doubt even worse.
APPENDIX:
If altruism is defined as working for the other person's need and not your own because you know how to be good, then altruism is a myth if we don't really know good. Guessing that we should serve others and seek nothing in return at all seems to be a poor basis for helping others. You will feel dehumanised in so far as you are not sure you even should. We need strong beliefs and facts to encourage us to work and keep working.