I agree, "evil" is a misconception, there exists no such thing as an "evil" person, in reality, just as there is no such thing as a "darling" person, in reality. But both expressions work as an expression of sentiment. When we use it we aim to communicate that we feel no empathy for such people (in case they are "evil"); they can without further ado be thrown in the dungeon. It is a dehumanizing construct enabling hate, same as calling people vermin, or monsters, but with religious connotations, exposing a will to exclude such people from the community (often for god reason), enabling going to war, or to exploit.

However, by removing empathy, we also reduce the possibility to understand the human motivations behind heinous acts (there always are), find solutions, build bridges, make truces, end wars. So maybe we should go lightly on the "evil" stuff, as much as possible.