I find that writers are usually nice people. Nice people have a hard time understanding nasty people, let alone liking them. Let alone loving them!
You must overcome this.
Because even though you deplore evil in real life, you must be able to embrace the evil mindset to write a good novel, especially a mystery or thriller.
Not to get all English-majory on you, but I remember a pertinent lesson from studying the early novel Gargantua and Pantagruel (Francois Rabelais) in university. To hyper-simplify what Rabelais tried to convey in that vast satire: to be a man is to be a dog (with a dog's disgusting habits and appetites), and the only way to fully be a man is to enjoy being a dog.
When you take this online writing course, you will learn how to create believable fiction characters and construct scenes with emotional depth and range.
There is our lesson for writing villains successfully: to be an author is to be a villain, and the only way to fully be an author is to relish being a villain.
Thus we must learn to enjoy playing in the dirt, oui?
Even if your story will not tell anything from their viewpoint, you really need to get to know your villains so they will act realistically and consistently. Brainstorming on your bad guys will definitely help your plot as well as your characters.
Reach into your own dark side for this one.
1) Spend some time remembering something awful you did that you were sorry for. The specifics are unimportant: remember how you felt when you were doing it. Jot a note or two.
2) Now remember something awful you did that you're not a bit sorry for. Feel that feeling! Jot a note or two.
Those two simple practices will instantly improve your empathy for your villains.
Now, must your villains be bent on destruction and murder 24/7?
Real villains in the real world often act like the nicest people ever. Ted Bundy worked a suicide prevention line while he was killing women who looked like the girlfriend who threw him over. Jack the Ripper probably had friends. That BTK guy—remember him?—had a whole family, friends, a church…
Your villains are merely people acting in their own self-interest, feeding their own needs—only with total disregard for the rest of us. That is where they differ from normal people. The truly horrifying thing is, they don't have to differ all that much, to be effectively evil.
I might add that believable characters are always a mix of good and bad; it's really just a matter of degree, and of course, perspective. The axe-murderer's mother will believe to her grave that he acted in self-defense. He will believe he acted in self-defense.
Which leads us to more depth: Think about your characters, and love them, in light of human failings like self-delusion, unrealistic expectations, secret yearnings—yearnings that can't possibly come true.
Enjoy the dirt, and reap the rewards!