Every villain is the hero of their story...even if that story somehow requires the murder of millions of innocent people for absurd reasons. Look at the real world. The worst war criminals that are rightly reviled, save by a lunatic fringe, didn't just wake up one day and decide to commit wanton murder for the hell of it. They were convinced that what they were doing was justified in some sick way. This doesn't make their actions any less vile. Indeed, it makes them more so. And even they had drives and lives outside of that (and often people they cared about, even if it was in a selfish way). Star Wars, like many fictional stories, tends towards dramatic portrayals, but real world evil is often quite 'banal'.
For me personally, I don't find card-carrying, sadistic villainy that interesting.
Enyo is my main villain character, and when I started, I essentially wrote her as akin to someone who grew up in a totalitarian environment like the Hitler Youth or IS (she was created by genocidal HRDs who wanted to use her as a living weapon to further their plans to go all Skynet on the galaxy). She did bad things in furtherance of what she believed was a good cause, she was hurt and manipulated. Jealousy and an inferiority complex also played a role (she's Sio's clone and grew up in her mirror's shadow).
When pushed too far (being forced to kill a sibling) she lashed out. But the crucial thing is, while Enyo turned against her makers, she'd absorbed a lot of their beliefs. Enyo is darkside, but not your typical hate-filled Sith. There's rarely any malice in what she does. She simply doesn't value organic life much outside of her siblings and some select people. Do your job well? She'll be aloof and distant, but treat you well. Screw up? You'll be painlessly processed into a droid or enteched and never mess up again. She cared about her siblings enough to let those who disagreed with her morally choose their own path...she's also enough of a control freak to spy on them for their 'protection'.
Enyo's a 'Dark Jedi' who thinks like an HRD most of the time. In some ways, her code is more like an unconscious twisting of Jedi ideals because she was raised in a monastic way. Her evil manifests less in hatred (she doesn't even hate Jedi, even though the Korriban City orbital bombardment crippled her), and more in the absence of empathy for the vast majority of sentient beings.
She isn't sadistic because that is inefficient. She respects foes who put up a good fight (this was, for instance, demonstrated in her duel with
Loske Treicolt
. It's one of the reasons she spared her, the other being that she feels a bit of empathy for another clone). She also helped TSE unleash a zombie outbreak on Mirial back in the day and her business model is that of a wardog. She's a horrible person who deserves death, but there were events that led to that.
So in short, when writing a villain, treat them like any other character. Establish motivations and goals. And, hell, stuff they do when they're not doing evil deeds. Not every villain wears black, monologues about the dark side, immortality and 'unlimited power'.