Evil exists. Evil believes that what it does is right. And Good believes what it does is right. Both are right in their frame of reference. However if Good makes the Evil realize that it is Evil then what will happen to Evil ? Will it become friends with Good? Will the self-realization make Evil good ?
This only works when assuming that there are moral facts in some way. Whether those are absolute, relative, or subjective doesn't matter here. But if no moral facts exist then we can't act morally wrong. If we can't act morally wrong then the label "evil" can't really work prescriptively.
Existence of moral facts is certainly defensible and at the moment a majority view of Anglophone analytic philosophers.
(With that premise, we could still argue that "evil" is conceptually unclear or doesn't exist, but I don't think this is a popular view.)
Evil believes that what it does is right. And Good believes what it does is right. Both are right in their frame of reference.
This isn't self-evident.
Firstly, we might differentiate between morally bad and evil. If bad were automatically evil then even bad behaviour because of ignorance through no fault of one's own would be evil. For example, if we are deontologist and somebody that thinks utilitarian does something which we find morally wrong but which they find morally right then we'd have to call them "evil". I find this unintuitive.
There are various contemporary theories which hold that evil is moral wrongness (or intent for moral wrongness) plus some special quality. For example a special motivation or a lack of motivation. The SEP has a nice article that covers some approaches. See here.
Secondly, (even if we still think that evil and morally wrong are the same) it could be that someone could act morally wrong while knowing that his behaviour is wrong. This only works under some metaethical views. If we think that knowledge of moral facts automatically motivates us to behave according to them then this isn't possible. Even with this another case is possible: someone doesn't know that what he's doing is morally wrong but 1) doesn't care about morality and 2) acts morally wrong. In that case we might "have evil" that doesn't think that what it does is right which is compatible with many metaethical views.
Thirdly, good, as in moral goodness, doesn't automatically have to believe what it does is right. At least, it's possible to defend that one can act morally right without direct concern for morality.
However if Good makes the Evil realize that it is Evil then what will happen to Evil ? Will it become friends with Good? Will the self-realization make Evil good ?
Beforehand I wrote about moral motivation. Some theories hold that moral facts are inherently motivating. Those theories are usually grouped under the label "internalism (about moral motivation)". So under such theories, if someone understands that their position is wrong then they will stop doing morally wrong acts. Then we can argue: if they stop doing morally wrong acts then they'll also stop doing evil acts. I think some theories of the concept of evil could criticize that last step.
But there's also "externalism (about moral motivation)". Theories under this label argue that moral knowledge isn't itself motivating. We only act morally right (assuming that our beliefs are correct ofc) in order to satisfy something. For example, we might act morally right in order to: avoid blame, avoid guilty conscience, get better social standing, act coherently, etc. Such theories will think that the question "Why act morally right?" makes perfect sense. This then means that someone might act evil in full knowledge of what would be morally good.