True evil, Lord Voldemort is a prime example of being evil. He is far more evil than Satan, or Darth Vader for example
Lord Voldemort is visibly a deeply evil and corrupted character from a very early age, with no real reason behind it, except for being left with no parents, but not all orphans are so demonically evil. He is just evil for the sake of being evil, he wants to cause pain and suffering both long term, and short term. Without a second thought he splits his soul into 7 (8 if you count the time he didn't mean to) pieces, to ensure that his reign of evil did not stop. He doesn't want to rule the world, he wants to influence it in his own image, for maximum oppression, mental instability, fear, anguish and pain. The way he feels towards un-pure witches and wizards are also very extreme, as it's racism. He believe's that all of them should be destroyed. Thats, the half wizard/witch humans, and humanity itself.
Satan isn't evil at all really, if you go for the original Satan, and not the Gothic pop culture version- Then Satan was the lord of music, who was cast of of heaven and into hell for questioning god. That's it, nothing more! And then we damn him as the lord of all evil for having free will? Tbh, I'd say god was more evil, for damning his life to a eternity of fire and anguish for having free will.
Darth Vader, isn't evil. He is weak, he fell to the lure of the dark side, and with no direct control over himself, he blindly did the emperors bidding. He was a good Jedi to start with, but was weak with the thought of revenge on the sand people. And not to mention he redeemed himself, overall not very evil