In the Season 1 finale of The Lord of the Rings: The Rings of Power, it was definitively revealed that Halbrand was the Dark Lord Sauron. This revelation was shocking to some fans and not surprising to others given how The Rings of Power hinted at Halbrand's identity throughout the season. Given the fact that the show pulled a red herring with the Stranger, it's not surprising that fans are suspicious of Sauron's true identity.
Considering the creative liberties Rings of Power is taking in order to tell its version of the events of the Second Age, Sauron's true identity has come into question by a few fans. Given Halbrand's good deeds and heroic actions, some fans have even theorized that Sauron impersonated the real Halbrand.
Sauron Isn't Actually Halbrand, According to This Theory
The theory suggests that during the eruption of Mount Doom in Episode 6, the real Halbrand was either killed, injured, or captured by Sauron. Sauron then took the opportunity to shape-shift into Halbrand to assume his identity and feign a serious injury, so he could be brought to Eregion to get close to the Elves and continue with his plans.
Sauron Is Too Evil and Too Powerful for His Own Good
The theory relies on the fact that Sauron is known to be a shape-shifter and a master of deception. In The Silmarillion, he took on at least four different forms during the First Age alone, his last form being a "fair form" that he took up again in the Second Age to deceive the Elves of Eregion into making the Rings of Power. This gives some weight to the theory. The fact that Halbrand performed good deeds earlier in the show, such as saving Galadriel in the Sundering Seas and fighting off the Orcs in the Southlands, is also used in support of this theory with the idea that Sauron would not go so far as to save Elves and Men, even to further his plans. Therefore, he must have come into the picture after Halbrand's great heroic act.
Halbrand Starts off Following the Same Character Arc as Aragorn
Halbrand is made out to be Rings of Power's Aragorn. He is skilled with a sword and rides into battle to save the innocent from evil creatures. He is a likable character from the beginning and the potential that Galadriel sees for him reaches the audience. Halbrand is a wandering man looking for work after having his kingdom taken from him. He has the potential to reclaim said kingdom from the forces of evil and be placed on the throne to become the next great and kind ruler Galadriel knows he can be. The only thing stopping him is his own shame for having lost his kingdom to begin with, so he's reluctant to try and reclaim it because he fears he's not strong enough. It's only after traveling with a hopeful and strong-willed companion that he realizes he might have what it takes to be a hero.
It's made clear to the audience at the beginning that Halbrand has as much potential as Aragorn did to become a heroic king. So it stands to reason that the Halbrand revealed to be Sauron at the end of the season couldn't possibly be the real Halbrand because Halbrand was just beginning his heroic journey to be king.
Rings of Power Makes It Obvious Who Sauron Is
Halbrand being killed to make way for Sauron to shape-shift his way into the Elves' good graces sounds wild enough to just work. However, the main problem lies in the fact that Rings of Power provides several hints throughout the season that Halbrand has always been Sauron. From his introduction where he saves himself and Galadriel by sacrificing the other humans to the Worm and up until he suggests to Celebrimbor that three Rings of Power should be forged with the Mithril instead of one crown, Halbrand's true identity was always lurking beneath the surface.
As a master of deception, Sauron would have the skills to charm his way into gaining Galadriel and the Elves' trust by saving a few humans and feigning being a man who only wants to work as a blacksmith and live in peace after the destruction of his kingdom. Much in the same way that the One Ring did what it had to in order to return to Sauron's side, Sauron always had the capacity to perform good acts so long as it brought him closer to his goal. In The Silmarillion, Sauron was in Eregion for two hundred years before the rings were forged. He is anything if not patient, therefore drifting on a raft in the middle of the sea with a bunch of humans and trying to gain the trust of a few blacksmiths and an Elf is hardly a price to pay in achieving his ultimate goal.
Sauron Is a Greater Villain Than He's Given Credit For
Sauron's intentions were always to forge the Rings of Power. His interest in finding work as a blacksmith was a hint at that. He knew that he would require the help of the Elves in forging the rings, so it was imperative that he stayed as close to Galadriel as possible. This eventually led to him meeting Celebrimbor and setting the wheels in motion for his plan to eventually create the One Ring. Posing as a lost king to a fallen kingdom was the perfect bait to prey on Galadriel's sympathies and get him a step closer to the Elves. Repeatedly putting off Galadriel's insistence that he is meant to be a hero and a king allowed Sauron to draw her closer to him so that when he did perform as a heroic king on the battlefield, Galadriel would have no reason to believe that Halbrand was anything but a destined king.
Were it true that Sauron took Halbrand out of the picture and took on his likeness, it would diminish Sauron's potential as a villain. Being too evil to trick his enemy into trusting him is not the type of villain Sauron is capable of being. The One Ring, which Sauron forged himself, was capable of deceiving both its wearer and the people around it into desiring it despite the heroes knowing the Ring should never be worn and could bring about evil. Even Gollum was able to deceive Frodo and Sam into following him into Mordor before attempting to take back the Ring. Halbrand being Sauron in disguise from the very beginning completely falls in line with what the Dark Lord is capable of.