Is Evil necessary ? (For what it is worth, some thoughts on the effects of words... )
The choice of words one uses to describe inner conflicts may be a subtle but critical point. I am referring mostly to connotations and possible associated meanings here, rather than strict dictionary definitions (denotations). The word "evil" (as technically accurate as it may be) may possibly not be the most helpful here, as it is a loaded term.
Speaking for myself, the word evil (or Evil) has associations of being extremely powerful, perhaps nearly irresistible or unbeatable, along with being related and similar to the word "Devil". As powerful more or less as "Good", as though it is some kind of dark Dionysian counterbalance to the Apollonian light. Or that evil is the inevitable Yin to the holy Yang, or something. (Both of which it definitely is NOT. But of course evil and wrong will almost always quickly lead to negative and painful consequences. This is a most important ethical and social issue, by all means.)
Personally though, whenever i have thought this way (that good and evil are somehow equally powerful or necessary) and acted upon it, i have gone astray into confusion. At times, there has been almost an obsession concerning evil/devil. Perhaps this is understandable, given the current world circumstances. (Disclaimer: this is NOT meant as a theological statement, affirmation, or denial). But as someone once said, "where attention goes, energy flows". I find it clearer and more effective to use words like wrong, bad, mistake, error, imbalance. They seem to have less baggage than the word "evil". Just a preference one may or may not find useful.
Breaking down the "wrong" into its component ingredients of ignorance, greed, and hatred, as Buddhism does for example, may shed light on how to recognize and neutralize it within oneself. Since most would agree that we can only control ourselves... and often even that is difficult. This is not meant to downplay the seriousness of the issue at all. It is meant more as a strategy to embrace the good in one's life.
One could compare the difference between right and wrong to two people cooking their dinner. One person is cooking it at an optimal temperature, and it is hardly noticeable except for a pleasant aroma. The other person cranked up the heat way too high, and there is smoke and flames which triggers the alarm. A real panic which naturally grabs the attention. An error in judgment leading to danger, but the basic elements are not essentially different than the first case. (just my two cents worth)