I do believe that evil exists, but not how most people think of it. I believe that evil is a state of mind and is a purely human construction, of which only humans are capable of being.
Evil, to me, is simply thinking that others (no matter who) are different, or more specifically, inferior to ourselves.
As soon as someone begins to believe themselves to be different, or better, to anyone else, then that is where evil stems from. Because in thinking someone else inferior to yourself, whether in shape, form, thinking, or in any other way, then you deny that person their equality to yourself and such thinking has always been the base for any act of “evil” you can think of from history.
It is by encouraging their citizens to think of themselves as better, or more moral, than an enemy, in order to justify their killing this enemy. A murderer allows themselves to commit murder because they think of their victim as being “inferior” to themselves, and so not worthy of life as they are. A racist justifies their prejudice by thinking the other person to be of an “inferior” racial stock. A slave owner justifies owning slaves because they are “inferior” and so do not have the same rights as other humans (in this case “humans” means other people who think the same as they do). The thief or conman justifies their stealing because they have more right to your possessions than you do. A businessman justifies ripping off his customers because his need for the money is greater than theirs. And so the list goes on...
All humans, of all societies, are inherently programmed to think of others as “different” or “inferior” in some way, and sociologists call this “Identity” and think it essential to the way that people see themselves. We all, at some point, think of “them and us” and, inevitably, “us” is the side of good versus “them” as the side of bad, and we can justify our mistrust and hatred of others through this “Identity”. Humans naturally seek groups that they can “belong” to, and as a consequence other groups that are opposite, and therefore an enemy to those groups.
Until we stop thinking in this way, release ourselves from this “identity” need and realise that all people, no matter who they are, what they do or where they are from, are the same, equal and have just as much right to their own lives, opinions and customs as we have, then evil will continue to exist and humans will continue to be inherently evil.
Anyone that thinks of themselves as being patriotic, nationalistic, or in any way different from other people is committing this evil and continuing evil acts.
One more thing that I'd like to add is that many people throughout history have equated deformity or ugliness with evil. I can 100% assure you that how someone looks does not indicate how good or evil they are. Any connection between deformity and evil behaviour is more likely to stem from the deformed person being abused by others because of how they look, and so may lash out and react to how they have been treated. (I wanted to emphasise this point because I suffered for many years from accusations that I must be evil because of how I looked).