Since Disney has declared the EU non-canon and thus nullified General Grievous' backstory, Grievous could now be a probable contender for a Pure Evil villain.
For those who don't know much about him, Grievous is a half-crazy cyborg warlord who leads the Droid Armies of the Separatist Alliance in the Clone Wars. While many Separatists believed the Galactic Republic was corrupt and sought to replace it with a better system, Grievous's only reason for participating in the war is so he can kill the Jedi.
While most of it happens off screen, Grievous has killed thousands of Jedi and keeps the lightsabers of the Jedi he's killed as trophies. He's pretty much the Star Wars equivalent of a Serial Killer.
In fact in the "Lair of Grievous", the braids of Jedi Padawans can be seen among his collection, heavily implying that Grievous isn't above killing children as well.
MORAL EVENT HORIZON
Grievous doesn't really have a stand-out moment where he crosses the line, he kind of starts out on the other side of the line to begin with. He does show several depraved behaviors throughout the Clone Wars though. He attempts to kill anyone who identifies as a Jedi, even if that person wasn't in his way, and also has a habit of killing his minions when he's angry (granted, his minions are droids, albeit sentient droids, but it still goes to show how little care Grievous has in other lifeforms, mechanical or otherwise)
In the old EU, Grievous had his brain altered by Count Dooku to make him more aggressive, effectively brainwashing him into being his enforcer. However, since the EU is no longer canon, this no longer applies and means that Grievous was acting of his own free will throughout the entirety of the Clone Wars. Its even mentioned in the new EU that Grievous voluntarily CHOSE to become a cyborg rather than being forcibly made into one.
Grievous doesn't have any redeeming or sympathetic qualities in any of the canon works he appears in, as I recall. He isn't very fond of his boss Count Dooku or any of the droids under his employ, doesn't have a sense of honor and frequently resorts to cowardly tactics to win battles, is implied to have slaughtered children as mentioned above, and is basically a crazy, warmongering serial killer put in charge of an army of killer robots.
Grievous doesn't get a whole lot of screentime in Revenge of the Sith, but he makes plenty of appearances in both the old and new Clone Wars cartoons and that's where his depravity and vileness really begin to shine.
Now granted, Grievous is no where near Palpatine levels of evil, but for what he is Grievous certainly stands out as utterly vile. He's definitely more evil than his boss Count Dooku and his bloodlust and single-minded genocidal hatred of the Jedi certainly makes him stand out from most other Star Wars villains, even many Sith.
I'd say he's a definite Pure Evil given what the Star Wars canon is now.