The "Dark Lords" of science fiction and fantasy usually don't succeed, which is why the select few that do tend to be immortalized in the antagonist hall of fame. One such villain is Emperor Palpatine, a.k.a. Darth Sidious of the Star Wars film franchise. However, let us not discount the notion that half of his success is probably due to his loyal apprentice, Darth Vader.
It has even been surmised that Vader had grown more powerful than Palpatine in the days of the original trilogy, machine body and all. Vader had the highest potential out of all Force-sensitive beings. However, he didn't exactly use such potential for good. Both he and his master were two of the most evil beings during the reign of the Galactic Empire. Both were directly responsible for some of the most unforgivable crimes in their galaxy, but who was worse?
Darth Vader: Wiped Out The Younglings
As soon as Anakin Skywalker became Darth Vader, he already committed pure evil. He killed dozens of Jedi students, mere children, at the behest of his master. The thought is stomach-churning, even for someone as lost as Anakin. He could easily have ordered clone troopers to just do it for him-- but no, he just had to do it with his own hands.
That's incredibly cold and ruthless, but it did seal Anakin's fate as an initially irredeemable Sith. We're only just getting started here, so brace yourselves...
Emperor Palpatine: Enslaved The Whole Galaxy
Palpatine could have easily stopped when he vanquished the Jedi Order. After all, not many in the galaxy can stop a Sith Lord, let alone THE Dark Lord of the Sith. Of course, Darth Sidious was a lot more ambitious than that. He wanted control of the whole galaxy and every living thing in it.
So, through cunning, power, and a clone army to boot, he made everyone's lives a living hell. He even replaced the Galactic Senate with the Imperial Senate, to ensure that there can be no political opposition whatsoever against him.
Darth Vader: Bullied And Maimed His Underlings
In the very first hour of the very first Star Wars movie, Vader has shown how intolerant he is of uppity subordinates. He thus demonstrated his displeasure by choking the wits out of everyone who dared lack faith and disturb him. That's what happens when you're too powerful. Hence, there are only a few people who dare order him or disagree with him safely: Grand Moff Tarkin, the Emperor, and Boba Fett (the certified badass).
You can imagine how bad it is working for or under Vader himself. Occupational hazards might include temperamental choking and/or death from administrative tantrums. In his own canon comic books storyline, he even maimed and dismembered some of his elite soldiers as part of their "training."
Emperor Palpatine: Wiped Out Most Of The Jedi
While you can argue that the Jedi had it coming and should have seen it from a mile away, not all of them deserved such a grim fate. Through Order 66, Palpatine ordered the clone army to kill all the Jedi and even destroy their temple. Only a few survived the overwhelming ordeal.
Killing most of the powerful Jedi has proven to be catastrophic for the galaxy. As we all know, a healthy balance of light and dark is necessary for harmony of the Force and all life in the galaxy. Darth Sidious was the only one who was able to disrupt such a balance, for the sole purpose of fulfilling his own aspirations.
Darth Vader: Flooded A City, Drowning Its Inhabitants
Vader's evil deeds extend far and wide, and not just in the films. In fact, some of his most deplorable cruelty was shown in the Star Wars canon comic books. While wiping out all the survivors of the Jedi Order, Vader fought Kirak Infil'a, a Jedi Master who survived Order 66. Vader was frustrated after not gaining any ground against the old Jedi Master, so he exploited his weakness: his love for his people.
Vader flooded a nearby city during their fight, forcing Infil'a to use the Force to try and stop the flood. While he was busy, Vader prepared to stab him in the back, but Infil'a pleaded with Vader to just kill him instead and spare the city. So, Vader killed him, but still flooded the city, drowning Infil'a's people.
Emperor Palpatine: Billions Perished Under His Tyranny
Maintaining tyranny is hard work, and the cost is often paid for in blood. Of course, Palpatine knows this and is more than willing to let others pay that price for him. Hence, through countless wars, massacres, assassinations, and slavery, the Emperor maintained his dream Galactic Empire.
It is worth noting that some of these atrocities were committed by his second-in-command officers, such as Grand Moff Tarkin (who was responsible for destroying the populated planet of Alderaan). Still, Tarkin was only able to commit such a crime under the Emperor's rule. Hence, we can safely attribute billions of deaths under the Galactic Empire to the Emperor.
Darth Vader: Betrayed The Jedi Order
Vader, even back when he was still Anakin Skywalker, was no stranger to biting the hand that feeds. We can recall that it was the Jedi who freed Anakin from a life of slavery. It was also the Jedi who taught Anakin the ways of the Force and how to use it for the good of others.
What did he do in return? He burned their temple, erased any written memory of them, and hunted everyone down. Granted, the Jedi Masters were not exactly kind or sympathetic towards him. Regardless, Anakin needed a lesson in gratitude or even common courtesy, even before he became a Sith.
Emperor Palpatine: Killed His Own Family
Sheev Palpatine, before he even became the Emperor or Darth Sidious, was already a dangerous rich kid. He wanted and valued nothing but power and ambition. Through his studies, he saw the Sith as a way to gain both. So he kept collecting Sith artifacts and studying them. Also, he learned quite a lot from his previous master, Darth Plagueis the Wise, whom he killed.
It wasn't just his master that Palpatine killed. He also sent his whole family and even their family guards to the grave. Initially, Palpatine only hated his father and had a burning hatred for him, though he did dislike his family; for him, they lack the drive for more power and influence. So, after killing his own father, he decided to wipe out the whole family tree as well.
Darth Vader: He Was A Bad Father
That isn't to say that Vader is better off as a family man compared to Sidious. In Revenge of the Sith, we can recall Anakin using the Force to choke his pregnant wife. We're guessing he hates children more than he hates sand.
Even after he became a lonely and sulky cyborg, his poor parenting never ceased. For one, he captured (and allowed the torture of) his own daughter. Vader also chopped off his own son's good hand. Lovely family reunion.
Emperor Palpatine: Is A Professional Liar
Okay, so basically, Palpatine was a rich megalomaniac whose only family or friend was his own power. The worst part? He probably didn't even need the Sith in order to gain that power he always sought and wanted. He was already a two-faced politician.
If they measured strength in the Force by the amount of lies somebody had told rather than their Midi-chlorians, Sheev Palpatine would be have been the Chosen One instead of Anakin. The lies he told just to gain power, both in the Senate and in the Sith, were enough to make a solid foundation for his Galactic Empire. Heck, he only had to lie to Anakin to create one of the most powerful Sith Lords ever.
The Emperor Is Worse!
There really is no contest when you compare the "Dark Lord" track record of the two. Neither deserved to be forgiven for their wrongdoings, but the Emperor's crimes were far more wide-reaching.
There is also the fact that Anakin wasn't born evil, and was only lured to the Dark Side through his loved ones. Meanwhile, Palpatine was already inherently evil as a child and wanted power for power's sake. In that regard, the clear winner is the Emperor, for showing the Dark Lords of fiction how it's done. He succeeded, after all... for a time.