Evil is a label we ascribe to an action or behaviour, the same as beautiful or disgusting.
It's a common mistake of humans to describe their own subjective evaluations as intrinsic properties of the object or behaviour in question.
For example,
"I find maggots disgusting" becomes "maggots are disgusting" - one implies a view, the other implies an intrinsic quality (Which is wrong) - we project our view of an action or behaviour onto the focus of our attention.
In short, evil does not exist - it's a view of an action behaviour or person - at best it's a gross simplification of a series of complex flaws in a human's development, at worst it's an idiots way of explaining the world.
Ultimately, you could say that anyone who intentionally commits harm against another living being is evil however they try to justify it.
I quite like that on a social level, the only part that gets more complicated is when you address the root cause.
If person A kills 10 children they are "evil".
If person B kills 10 children (but was abused & tortured for 10 years by his parents causing him to have a mental breakdown) - are they stil "evil"? or broken?.
If person C kills 10 children because of a series of seemingly average events which caused the erosion of empathy, disconnected from reality & violent urges (due to a neurological flaw/genetic defect) are they still "evil"?
At what point when you example the root cause of human behaviour do you simply label it all as "human "X" responding to a combination of genetics & environment causing them to behave in "Y" manner"?.