I'll just come out and say it, my past has left me with a very complicated moral compass. I offend people very easily when discussing the concept of right and wrong and personal responsibility to social contract.
It's like you said you have to first define "Evil". The simplest answer is a literal sadist is evil as they enjoy harming others. Then you have to form some kind of metric as to the "line in the sand" of what constitutes sadism to a point as evil. It's a great question, just nearly impossible to simply define evil. Even in a purely religious context in scripture it's all very blurry.
Here's a somewhat NSFW written example:
A man and a woman well above consenting age meet and form a sadomasochistic sexual relationship. The man derives sexual pleasure from hurting her, she derives sexual pleasure from receiving it. Is he evil for being sadistic if she consented and sought someone like him out? Is he taking advantage of a mentally ill woman? Are they both mentally ill?
We have a videos of people dying thread, where we openly mock and make light people on video dying. It sounds evil in that context but, it's all subjective. If we had some kind of emotional or empathetic connection to the person dying in that video we'd probably see anyone laughing at that video as evil.
It's all subjective. Here's a man I personally consider "Evil". Former District attorney Mike Nifong
As far as I know Nifong never shot stabbed or even beat anyone. He was a district attorney who promoted false rape allegations against some fraternity kids under the pretext this alleged gang rape was also racially motivated. The accusing party was clearly lying when DNA rape kit came back with DNA of 5 random guys none of which were who she accused or even at the scene of the alleged crime. Her story never made sense from the very beginning and she only claimed it to avoid arrest herself for some petty crime. Nifong fully aware knew this and conspired with law enforcement to rally people to re-elect him District attorney under the context these college kids gang raped a black woman and he was a tough as nails DA on racist rapist privileged white frat boys. Needless to say it all fell apart my point was the man wanted to trade ruining the lives of some stupid college kids to further his career. There are even more details to this story that make it even worse, point being this man meets my criteria of evil. This was know as the "Duke Lacrosse Scandal"
I think it's worth a mention the united states criminal justice system has sentenced many people to death, some of which had surprisingly low IQ's. It makes you wonder if they even had the mental capacity and understood striking someone in the face with a heavy object could kill them? Is it more akin to vehicular manslaughter where unintentional death was the result of negligence?