Evil really does not exist; actually in existence is only that which is good, and the absence of that which is good. The bad, or sin, or guilt, or whatever in this direction, is merely a confusion of language; our unverified implicit assumption is that the conceptual opposite of something which verifiably does exist, must necessarily also exist. This is not true; it either may or may not exist.
This is understandable analogously with Light and Darkness. Physically, light truly exists; as elemental particles/waves (photons) in motion as electromagnetic radiation; which can be measured in the laboratory, or sensed as sunlight hitting the skin which results in a suntan. Physically, darkness does not exist; there are no elemental particles of darkness; darkness cannot be measured or sensed. What does exist in various places to various degrees is the relative absence of light; which for language communication convenience we label with the term "darkness".
Only for the convenience of communication in language, only for the convenience of manipulation of thoughts, do we make such conceptual abstractions of both light and darkness in language; and furthermore define these verbal abstractions of light and darkness as polar opposites. At this abstraction level, either one can be understood as the absence of the other; and convenient productive manipulations of thought commonly occur without it being necessary to keep in mind that darkness has no actual existence.
The usefulness of these abstractions is illustrated by the "rising of the sun" concept. The sun doesn't rise, the earth rotates. For eons humanity conveniently arranged its workdays without difficulty arising from the inaccuracy of this notion. Only when people wanted to deal directly with this "rising", rather than with the results of the rising, did problems arise (and the science of astronomy develop).
Likewise for light and darkness. So long as we deal only with the results of either, there is no difficulty with these abstractions. But when we attempt to deal directly with darkness, rather than the results or effects of the darkness, then a serious problem arises. Because darkness has no actual existence; darkness is merely a generally convenient conceptual abstraction signifying the relative absence of light. Light actually exists. Darkness actually does not exist.
Exactly the same holds true for evil (or badness, or sin, or guilt, or anything else in this direction). Evil is merely a generally convenient mental abstraction signifying the relative absence of good or goodness. So long as we are dealing only with the results or effects of either, no difficulty arises. But when we attempt to deal directly with either, it becomes crucial to determine whether both actually exist, or one is merely a mental abstraction signifying the relative absence of the other.
Reports throughout the millenia vary; theories are not of much use; everyone is essentially left to determine the matter for themselves. But all infants, and any spiritually-sensitive adult can affirm (though they generally cannot effectively explain) that goodness actually does exist; that goodness can be actually directly sensed inwardly as certainly as sunshine can be sensed outwardly; sensed as a type of radiation (whether from life itself or whatever). And evil or badness cannot thus or otherwise be actually sensed. All that can ever be actually sensed in that direction is the relative absence of goodness.
Thus any search for (in order to identify and/or destroy) any source or embodiment of evil (or badness, etc.) is utterly doomed to failure, since such things have no actual existence. (Likewise, all notions such as: Satan, the Devil, witches, evil spirits, etc.) Consequently, to be effective in leaving this world a better place than how one found it, one must quite truely light a candle rather than curse the darkness.
There is no Darkness. There is no Evil.