Rings of Power, like all adaptations of Tolkien, spent much of the series exploring the concepts of good and evil, and how certain characters exist in the boundary between the two. Characters like the Stranger, who could easily be mistaken for Sauron, have to make a choice whether to turn towards the growing darkness of the world, or to use their powers for good. He has made the choice to protect those who need protecting.
During the last episode of the series, this same choice was presented to Galadriel. She finally discovered Halbrand’s true identity as Sauron, and he offered her the chance of joining forces and ruling Middle Earth together. As Lord of the Rings fans knew she would, she refused him, turning instead to fight against the darkness. But what would have happened if she had accepted his proposal?
The real basis of this question lies in whether Galadriel has more potential of turning Halbrand towards peace, or whether Halbrand has more potential to corrupt Galadriel towards destruction. Ultimately, their union could only ever result in those two paths, with one converting the other to their own mission. When Halbrand first admits his real identity, he traps Galadriel inside her own memories. In doing so, he tries to explain to her that he actually wants to heal the world, rather than damage it. So there is a possibility that with the right guidance, he may have been able to rule the world honorably and justly.
But, there is clearly also the possibility that Galadriel would have become power-hungry and twisted instead. It has already been demonstrated throughout the series that she has unquenchable anger, which she describes as "a tempest" within her. This, if placed in the wrong hands or not kept in check, could lead her down a very dark path. Adar hints at this when he tells her that she should look in the mirror to find the evil that she seeks, after she threatens genocide of the orcs that he loves so much.
Throughout the series, there are no examples of Halbrand doing kind deeds, unless it is in order to manipulate or control other characters. He abandons his shipmates to the worm in the Sundering Seas, beats the black-smiths in Numenor, and fakes an injury that has hidden implications. This suggests that he doesn’t have virtue or morality within him. Rather, he only uses the appearance of these things in order to achieve his own purposes. It is therefore unlikely that Galadriel would have been able to save him or turn him towards the light if she had agreed to rule beside him. She even tells him this herself when she whispers:
“No penance could erase the evil you have done.”
There are some characters, especially in Tolkien’s world of clear good and evil, who are beyond redemption. And in their hearts, these characters don’t actually want it. They believe that they can re-design the world better than the original creator, even if it means killing millions along the way. Having already helped Morgoth destroy so many things, Halbrand is one of these characters.
On the other hand, there are many examples of Galadriel being fairly ruthless. She becomes so lost in avenging her brother that she cannot see the harm she causes elsewhere. The connection that she had with Halbrand, which could even have led to them finding love in different circumstances, would have quickly led her into the temptation that already exists within her, bringing about her downfall, and the subsequent obliteration of the world as it is currently known.
With Galadriel's power and knowledge at his side, it would have been all too easy for Halbrand to tear apart the fragile alliances that exist between elves, dwarves, and men, and instigate all-out war among them. He wants to rule them to the point where there is no joy, no freedom, and no growth left within them. By using Galadriel as his queen, he may actually achieve his goal, especially now that she possesses one of the three elven rings of power.
This is a theme and a fear that carries on all the way to The Hobbit and the Lord of the Rings. Galadriel continuously walks this line between good and evil, as is shown in the scene when she almost accepts the one ring from Frodo, and turns into a horrifying green version of herself, a being of terrible power and dominion. This is the vision that Halbrand shows her of herself in the water, and it is the reason that she ultimately turns away from him. Galadriel knows that if she had chosen to rule Middle Earth beside him, she would never again be free of the evil that already exists within her, and within us all.