Like you, I have long been perplexed by the concept of evil. It may be a perpetual question for all human beings.
Since I am a secular person whose moral system is based primarily on the values of commerce, the definition of evil has been a hard won insight.
My definition is straightforward: evil is any circumstance where a more powerful party takes advantage of or harms a significantly weaker party.
This is the case where an adult abuses a child. This is the case where a strong person abuses a weak elderly person. Where a person with a gun threatens a person without one.
Evil is where a strong government persecutes an individual member of the society without providing any protections. Evil is where a person or party is defenseless in the face of an aggressor.
I have not been able to think of every circumstance where evil is evident in a secular world. If there are circumstances that do not fit my definition please let me know.
I do not consider evil to exist in Manichaean terms as the opposite of good. It exists in circumstances where the relative strength of both parties is evident and the malice of intent is clearly on the side of the stronger party.
Evil is not tragedy as in the case where a child dies before it's parent.
Evil is not something one brings upon one's self.
I think that evil is a concept arising out of human biology. Humans are expected to defend and protect their offspring. Therefore the violation of this biological standard becomes a moral issue.
Clearly when one animal eats another, or even eats plants, there are differences in power but evil remains only a factor in human culture.
Evil is any circumstance where a more powerful party takes advantage of or harms a significantly weaker party.