Some time ago, I created a character who was extremely disturbed, extremely violent, extremely narcissistic and completely delusional. Sounds like a charmer, right? I gave my story to one of my readers and the feedback was that they didn't like my character, who I'll call Dark, and didn't want to read Dark's scenes.
I was disheartened by this. After all, isn't it a writer's job to make their characters connectable to the reader, whether the character is good or bad? Yes. Unfortunately, there was nothing about Dark that was redeeming, nothing nice, nothing that the everyday person could relate to. Dark was a wretched human being. It did, however, make me as the question: how does one go about making bad character likeable?
A good friend once said, they need to be interesting. Great advice. People want to be able to understand your characters and feel they know them. They want to have something that connects them to the antagonist, even if he/she is a beastly demon bent on destroying mankind. Give them a back story and a reason why they do what they do. It may that they were just born that way, like the human killing demon. To make our demon interesting, they may, in contrast to their desire to kill humans, also find humans fascinating. Or they may enjoy reading books.
Another suggestion is find something normal about your antagonist. If they're experiences are too far removed from the reader's, the reader may not connect. Our demon likes to be well groomed and picks food from his teeth to keep them clean.
The stereotypical bad guy who only does bad things leaves your antagonist two dimensional. Remember, your antagonists have families and friends and people (or demons) they see every day.
It helps to go back through your character's childhood. Were they always bad? Did something happen in their life to make them bad? I believe both nature and nurture can play a role in why someone might do bad things, so you're evil characters don't always have to have come from an abusive home. Dark was from a very normal, happy family. He was simply a born psychopath.
Find the things that are dear to your antagonist's heart. What is it they care about? Besides killing, do they have any passions? Do they have any favourite things/possessions? Do they have morals? Do they have a sense of humour?
We often label bad people as monsters. My advice is to keep delving into what makes your evil character tick until you find that one thing that allows you see them (even the demons) as human.