When people would get the ring in the Tolkein series, they would envision their rule of middle earth. But why want to rule? There must be a motivation behind the want to rule. People want to rule because they believe their vision beats the hell out of what is. Pride in the idea that their soul is willing and flesh is weak, but with the ring their flesh will be made strong. Unfortunately it makes even the strongest of souls weak and the pride shifts towards the flesh. Suddenly they have the power, but not the will, instead they conquer to possess rather than to free. The more believable the villain's argument, the insanity plea will only get you off 1 percent of the time, the easier it is for the audience to relate. If the audience has some empathy for the villains actions then finally the villain is nolonger just a prop, the villain becomes a character on the same level as the others in the story. That is why characters like Magneto and Darth Vader are so classic. They aren't just the cut out prop character preaching to the heros before he circle saws them in half.
Just look at the source material for these Zelda games. Before the Arthurian lore there was Beowulf. Beowulf's first monster, Grendal is a memorable character not because of the horrible things he does. Grendal is great because at the end of his fight with Beowulf he rips his own arm off to get away from Beowulf who has a hold of him. The wound he inflicts upon himself is what kills him. The character who was a mass killer and man eater showed fear of death, pain, loss of freedom, and much more with character depthening actions.
Look at Jaws. The shark is an animal right. An eating machine. But that isn't scary, because real sharks are eating machines. They don't think, they don't plot, they don't have malice. But the shark in Jaws is more than animal. It does what so few animals have ever done in the wild. It kills and kills and kills and kills. Not because it's what it does or must do to get by, but because it wants to. It wants to kill another person. We don't need a monolougue to tell us the shark wants to kill the three men on the boat. Spielberg spells it out with action and he does it better than anyone. Jaws is scary not just because of the way it is done or that its a shark. Jaws is scary because behind those teeth and black emotionless eyes ticks the mind of a human. That is what makes a monster.
Even Satan is not ''evil.'' The character's sin was pride. Pride got the greatest angel in heaven kicked out. He wasn't eating babies. He just realized he was better than everyone else. After that the character is demonized and cast out. What would you do? What would you do if god gave you enough rope to hang yourself with, but not enough for your feet to touch the ground afterwards and you knew he knew beforehand. Wouldn't you then try to destroy every consecutive creation. But if the myths play out according to the rules, god creates Satan knowing Satan will have pride and wage war, knowing Satan would be cast out, knowing what he was going to create afterwards. Thus the creation of Satan is for the purpose of causing the first man and woman to be cast out of Eden. Not so he could have been the greatest angel. God made him so great in the text for the purpose of creating this figure who would be capable of reasoning his sin and causing himself to be kicked out of heaven so there could be an enemy to prove god right. So even Satan, the prince of darkness, is not evil, he simply flawed. He was impatient and prideful and became so much more because of it.