Sauron's evil grew from his original servitude to Morgoth, the world's first Dark Lord, into his own obsession for "mastering" all things in Middle-earth through wicked means. After Morgoth's fall, Sauron was fully autonomous and committed, for two Ages, to gaining tyrannical control over Middle-earth.
Sauron became Morgoth's capable servant, helping him in all the "deceits of his cunning". By the time Elves awoke in the world, Sauron had become Melkor's lieutenant and was given command over the new stronghold of Angband.
Melkor is the most powerful of the Valar but he turns to darkness and is renamed Morgoth, the primary antagonist of Arda. All evil in the world of Middle-earth ultimately stems from him. One of the Maiar of Aulë betrays his kind and becomes Morgoth's principal lieutenant and successor, Sauron.
Actually Sauron was a originally a very gifted craftsman of Aule, and step-brother to Saruman. However Morgoth later corrupted him with promises of power, teaching him of evil magic to dominate and rule over others; ultimately making him his lieutenant.
Defeated by the Host of Valinor in the War of Wrath, Morgoth was banished from Middle-earth into the Timeless Void, though it was prophesied that he would one day return.
Morgoth, the first to reign terror upon Middle-earth, was more powerful than Sauron and commanded an army of dragons and Balrogs. While both Morgoth and Sauron were cunning, Morgoth's rule and terror lasted longer and his corruption spread further, making him more formidable in the Lord of the Rings universe.
Bloodlines are very important in Tolkien's stories, and it makes a lot of sense that Sauron would fear the great-grandson of the man who cut off his hand. Aragorn carried the blood of Numenor with him. His genetic connection to the heroes of the Second Age established him as a dangerous opponent of Sauron.
After the downfall of Morgoth, Sauron continually strove to conquer Middle-earth throughout the Second and Third Ages. In the Second Age, under the guise of Annatar, he deceived the Elves of Eregion, who under his guidance had created the Rings of Power, whilst he secretly forged the One Ring in Mount Doom.
"So I think in his pitch to Galadriel, it cannot mean that he loves her or that there's any kind of romantic relationship. There should be no ambiguity around the fact that Sauron is evil — he's terrible, and he's using Galadriel to enhance his power."
The Balrogs were Maiar corrupted by Morgoth during the creation of Arda, who cloaked themselves in shadow and flame and carried whips and swords. Famed Balrogs include Gothmog, slain by Ecthelion, and Durin's Bane, slain by Gandalf.
Sauron was the most powerful of the Maiar—primordial spirits created to help the Valar first shape the World. His original name was Mairon, which means "the Admirable," and as an immortal spirit born before the world's creation, he was able to perceive the Creator Eru Ilúvatar directly.
However, the mere fact that the Valar didn't prioritize perusing Sauron proved he was a much lesser threat than Morgoth. It made perfect sense because Morgoth was a Valar, and Sauron was only a Maia. So, while Sauron was evil, Morgoth was clearly much worse.
Isildur used a broken sword to cut the Ruling Ring off Sauron's hand. Sauron was fatally wounded by Gil-galad and Elendil as Sauron killed them. Gandalf took the job of blocking the Balrog - and he was barely able to defeat the Balrog (in fact, he died in the process).
Sauron was originally a shape-shifting fallen angel and could take on various forms, including a vampire and a werewolf. Sauron's final form was terrifying and demonic, reflecting his own hate and evil, and he did not wear armor in the books.
In his notes, Tolkien remarks that Sauron saw Galadriel as his equal. Add that to the fact that she's untrusting of Sauron (yes, even when he's in his Annatar guise) and an unwavering, single-minded force, and it's more likely than not that Sauron feared Galadriel — or the potential she held to upend his plans.
Vickers' comments imply that Sauron's intentions with Galadriel were primarily political in his dealings with the elves and that he was using her to elevate his power as a ruler over middle earth.
Additionally, while he was in his weakened state, Galadriel could hold him off through the power of will alone. However, if Sauron had his One Ring and faced off in a battle against Galadriel, it's unlikely that there would be much she could do.
Thus, Galadriel's dark form in Jackson's The Lord of the Rings personified what would have occurred had the Lady of Lothlorien given in to temptation and taken the One Ring for herself. She would have succumbed to evil because the Ring was indomitable, even with her power.
Eventually, Morgoth was bound in chains by the Valar and thrown into the Void, leaving the permanent damage his evils had done, and his former lieutenant Sauron, to trouble the world. One day, according to a prophecy, Morgoth will rise again in great wrath, but he will be destroyed in the Dagor Dagorath.
She had no role in the major wars of the First Age, believing that defeating Morgoth was beyond the power of the Eldar; when Morgoth was defeated by the coming of the Valar out of the West she, having played no part in the earlier Kinslaying, was offered safe return to Valinor.
For that matter, the Valar didn't have the authority to remove another Ainur's power. If they had had that authority, they would have stripped Morgoth of his power, rather than going to war with him so many times.
Sauron would continue to spread Morgoth's teachings after his defeat, and ultimately became the new Dark Lord in the Second and Third Ages, but Morgoth himself would never return to the kingdoms of Middle Earth.
