You realize that 99.9% of all "evil" characters have redeeming qualities right? And almost no evil character thinks of themselves as evil. Bad guys are always trying to do what they think is morally right in their own heads (Obviously Anakin thought killing innocent kids was the right thing to do, but its still wrong and an act of evil). Often turning to the dark side (or becoming evil) for characters is often because of something tragic or unjust that happens to them so they use that to rationalize their decisions.
Sephiroth (Final Fantasy)- Shinra used and abused him, so he goes on a killing spree
Vader- Palpatine tricked him and he felt abused by the Jedi, so he goes on a killing spree
Thanos (Marvel)- Wanted to bring order to the Universe, so he goes on a cosmic killing spree
Galactus (Marvel)- Just wants to get a nice meal, so he eats planets (and everyone on them if they don't flee)
Loki (Marvel)- Odin lied to him about being an Asgardian, so he kills a bunch of people on Earth
Magneto (X Men)- Family killed by Nazis, humans have a vendetta against him, so he tries to kill a bunch of non-mutants, kills some mutants
Raz Al Gul (Batman)- His family was murdered, wants to cleanse Gotham so the good people can start over, so he murders a bunch of people
Red Hood (Batman)- Pissed that Batman let him die and didn't avenge his death. (Raz Al Gul brings him back from the dead after Joker kills him), he kills bad guys... I guess he's more of an anti-hero than a villain.. shrug
The Templars (Assassins Creed)- Think that if they have the relics of Eden they can save humanity... so they murder a bunch of people
Shea (Assassins Creed/Templar)- Betrayed by the Creed, so joins the Templars against his better judgement, so he assassinates ALOT of people.
Dracula (Castlevania)- His human wife is burned at the stake by humans, so he decides to end all life on the planet in the most horrific manner ever.
I'm sorry this is a lot of western comic/game references... there are a few characters that I can think of that are just evil for evil's sake. Like Heath Ledger's Joker in Batman or Evil/Mr. Zorg from 13th Element. But I actually think pure evil characters with zero humanity are rare in fiction and usually make fairly boring characters.