This is based on this question; link

I think that there are a number of componants to considere when talking about what makes a character truly evil. For me it's about awareness and how the person is raised. As well as empathy and age.

By awareness I mean; a truly evil character is in full control (no mental instabilities). They know completely how wrong the action is and do it anyhow. In other words motivation is a huge component too; a character that firmly believes that he or she is doing the right thing isn't truly pure evil. Because this person really thinks that he/she is in the right.

Characters like Kuvira and Morgause fall under here. Morgause (as a baby) was ordered to be killed. Why? Because she had magic--basically because she was a witch. Because of this she tried to kill the king and his supporters. She was trying to protect herself and her kind. Kuvira (though a dictator) started out simply trying to unite a bunch of cities and towns to make them more organized and less susceptible to attacks. Somewhere down the lines she went on a power trip. (POSSIBLE SPOILER) In the end she admitted that she was in the wrong and really was just trying to help. These two--Kuvira especially--aren't evil in my eyes. A truly evil character would have insisted that 'yes' I was totally right. Kuvira owned up to it and decided to go down (get arrested) without a fight). In other words she has a sense of morality and realized that she got carried away.

Bellatrix is closer to pure evil; she kills and tortures for fun. This is where stability and awareness comes in though; Bella is very obviously mentally unstable so I still hesitate to call her pure evil. I think that Bellatrix blurs the line between right and wrong. And due to her childish nature (in her own twisted way) not even realize that's she's doing wrong. Umbridge is a better example; this woman tortures kids to get kicks and she is fully aware of it. Bellatrix's brain kinda just releases all kinds'a wild endorphins when she hurts people. Umbridge's doesn't. Umbridge hurts people out of greed and to get what she wants. She is well aware of how it will impact people and is in full control but does it anyhow in order to self-preserve. I discussed this specifically here; link

The next major constitute is empathy; are they sadistic? Are they merciless? If they get a thrill out of doing awful things and/or have no sense of mercy they are probably pure evil--especially if there's no mental hiccup. If the character has second thoughts or seems like they don't want to do the thing then I think that they aren't pure evil. I'll take this from my article, 'Types of Villains' link There are two types of villains that I would say aren't pure evil; the desperate and the unwilling;

"The Desperate

Similar to the wimp, but not necessarily a wimp, the desperate villain is the villain who does evil or becomes evil out of desperation. For instance the bad guy who is killing for money because he/she doesn't want to lose his or her home. Or because he/she has to feed the children. Or in general this person does horrible things to protect loved ones. Often times this is what tragically leads to the loss of the loved one.

Examples: Rumple (Once), & Tucker (FMA)

The Unwilling

Closely related to the desperate, but not like the wimp, this is the character who has no choice. Maybe his or her family is being held captive. Maybe he/she will die if he/she does not commit acts of evil. This character is usually forced to do evil by the true villain of the story.Sometimes this person is being used and he/she doesn't even know it. It can also just be a person who is evil due to the circumstances--like participants in the hunger games. They don't want to kill but they are forced to do so.

Examples: Sherry (Fairy Tail) & Practically all of the Saw characters"

These examples indicate that the character has a functioning moral compass but is lost, confused, trapped, or (as mentioned) desperate. They have second thoughts after or throughout the deed. Regina Mills is also a good example (season 7 spoilers); she cast another curse and actively tried to intervene with her own son's love life. The curse is evil--it was evil is season one. However, Regina was effectively backed into a corner--if she didn't cast it said son would have died. Evil action, greater good.

As for the textbook definition of empathy and mercy. I bring Bella and Umbridge back. Both of these characters show no mercy nor any empathy. They hurt people for laughs and they don't feel bad. Their victims are dehumanized.
Azula is another example. She is relentless; if you fight hurt you're finishing the fight, no surrender. She has a pretty decent lack of empathy (but at the same time I don't call her evil because she's pretty unstable--more on that to come). Which is why I'll point back to Umbridge as the big example. No mercy and no feels.
Going back to Regina; she isn't pure evil because even at her worst she still felt bad about what she did. She killed her father but was crying the whole time. And after she redeemed she cried harder. As a villain, she saw her victims as human and it haunts now as a good guy.

The third piece is how the person was raised. If they were raised without love, compassion, and/or a role model then they get more sympathy from me.

Once again I bring Bella and Umbridge up. Bellatrix was raised to hate muggles and half-bloods. It was ingrained in her since she was born. I would imagine that she'd have been disciplined harshly if she questioned that, after all her sister was disowned for marrying outside of pureblood. Basically Bellatrix didn't have a chance because literally everyone in her family and probably the neighbors were teaching her that muggles and blood mixing are bad. Umbridge has a squib (non-magical) brother and she still hates half-bloods. She is related to the Cracknell family, a family of muggles. And she hates muggles. Obviously having at least one muggle family member, she wasn't raised to hate them--unlike Bella.

I also bring Azula back into this. She was bred and raised for war. She was basically a human weapon. Her father drained her of empathy to make her better suited as a child-soldier. Because of this any positive role-models (Ursa, Zuko, and Iroh) were off put. They saw what she was becoming but took the, "I ain't touching that with a 10 foot pole" mentality instead of trying to intervene. Honestly even if they tried Ozai probably would have put up one helluva a fight. Like Bella, this girl didn't have a chance. Hence why I say she isn't pure evil. Unlike a character like Umbridge, she was raised to do (at best) morally ambiguous things.

Lastly, if the person is a child (depending on how old) they may or may not understand why the action was wrong or how it impacts other lives. Even if they are; the person is still a kid.

Overall, my go to example of a not-evil villain is Azula. As mentioned prior, she has done a lot of bad things but she's a 14 year old abuse victim that had no positive role model in her life. In other words she meets all of the criteria for not being truly evil. Her dad basically raised her as a human weapon--he raised her to be manipulative and use people. Bad thing was still bad thing, but I don't think she's evil I think that she's very hurt and very misguided. The kind of character who, with intervention, could turn out okay because she isn't inherently evil. Which come to think of it is another point; the natural inclination. Was the character born evil? If not then they aren't pure evil especially if they were a good person for a great portion of their lives. With Azula I'd also like to point out that, even though she's a hella good antagonist she doesn't go out of her way to do bad things. She has had many oppertunities to kill innocent people. The only person she took a killing shot with (before haveing a complete pshycotic break) was Aang. Aang, being the character that could have her side of the war defeated. Everyone else (lmao this is completely contradicting what I said above) she showed mercy too. She had a bloodless coup; there were about four powerful war generals that she had imprisoned instead of executed. On top of that; her attacks were strictly military.
All of this is why I feel like she is not evil. Not at all. Is she a bad person; lil' bit. But I think there's a difference between bad and evil. Bad can be helped and healed. Evil is tricky and doesn't want to change.

Regina Mills is my other go to example. She had a very similar story; her mom as well as Mr. Gold groomed her to be evil so they could achieve their own means. She also resisted the darkness until she was finally pushed over the edge due to very careful manipulation. She did horrible things. But after many years she was finally shown love, real love, by one person and that one person made a world a difference. She ended up turning good and trying to save other people from the same fate as she. On top of that she went very out of her way to right as many of her wrongs as she possibly could.

Maybe what I'm getting at more or less is that characters like in the example above are 'evil' sure but they aren't pure evil. Azula is the exception; she's never killed people. And all of her crimes were committed on the battlefield.

So what then what does make a character truly evil? Umbridge. Be like Umbridge; she was not raised to be evil. She has no clear mental instabilities. She's selfish and power hungry and gets a thrill out of being needlessly evil. She also doesn't show mercy unless it benefits her. And she's a grown ass woman. We also didn't see any indication that she went through anything traumatic to shape her into what she is.