Warning: spoilers for Steven Universe, now streaming on Hulu.
Pink Diamond, one of the four Diamonds in Steven Universe, is the one viewers get to know most intimately. Her character arc is presented in reverse, starting at the end before working backward to show why her growth was so necessary in the first place. Viewers finally see her at her absolute worst toward the end of the series.
All of this can leave audiences with a bad taste in their mouths regarding Pink Diamond. Was she beyond redemption? Let's break down her actions and the measures taken to redeem herself to see if Rose ever truly made up for her past sins.
Sins Past
Steven Universe Future and Steven Universe: The Movie showcase the start of Pink's arc, and her lowest points as a character. The impression of the younger Pink is that she's a spoiled brat with an uncontrollable temper and self-centered intentions. Pink crying to Yellow Diamond for a colony of her own in the episode "Jungle Moon," ultimately throwing tantrums when she doesn't get her way. She's shown to be entirely self-centered, not necessarily out of malice but due to a lack of understanding and empathy.
This is most clearly illustrated when she permanently damages Volleyball (Pink Diamond's original Pearl, later given to White) in a fit of rage. This is one of the few cases where Pink is shown her powers in a destructive fashion. When the Diamonds eventually relent and give her a single colony, she immediately abandons everything to go run it, including her playmate, Spinel, who she abandons in a garden for thousands of years, never returning to.
At this point in her character arc, Pink is essentially a God who can't see beyond her own self-interests. The value of other lives doesn't register to her.This isn't too far off from how the other Diamonds act; until the end of the main series, they're so caught up in their own desires and wants that they seemingly lack empathy for any beings lower-ranked than themselves.
Becoming Rose Quartz
Pink starts to mature, albeit in a rocky fashion, when she arrives on Earth. She starts construction to turn Earth into a gem production facility, only to want to socialize and hang out with the Amethysts and Quartzes she creates in the Kindergarten. This is where Pearl suggests Pink disguises herself as a Rose Quartz.
Because of this, Rose is able to step outside of her role as a Diamond and see the world through new eyes. This decision forces her to see the beauty and value of organic life on Earth, shocking her out of her prior mindset. While this revelation doesn't redeem her in and of itself, it does trigger the start of her redemption. She tries as Pink to convince the Diamonds to spare Earth, but they ignore her. They create a human zoo for her, seeing humans less as valid life forms and more like pets, much like Spinel was.
This forces her to properly assume the mantle of Rose Quartz, essentially starting a faux-civil war against herself. One one side, the freedom-fighting Crystal Gems, Rose Quartz and Pearl (who was also in on the whole charade). On the other side, Pink Diamond -- again, herself.
This changed once she met Garnet, which showed her that Gems could evolve beyond their castes. She understands that Gems are capable of changing, much like the life on Earth she adores. At this point, the faux-war becomes a genuine war of liberation and ideology, less about just Earth and more about breaking free of the caste system she had become a part of. Even now, however, Rose was still a morally questionable character, bubbling the dedicated Crystal Gem Bismuth in order to protect her privileged identity as Pink Diamond.
Eventually, she decides to end her existence as Pink and become Rose Quartz forever. This is done by faking her own death with Pearl. Pearl, however, is ordered to never speak the truth to anyone. The Diamonds respond by trying to scourge the Earth, leaving countless fragmented Gems corrupted on the planet.
Secrets and Lies
Rose's big issue is that she was incredibly secretive in order to maintain her rebellion. Because she is just Rose Quartz, she couldn't go rescue Spinel because that would result in her secret identity being exposed. Rose, after all, wanted to remove all traces of ever being Pink. She couldn't go apologize to Volleyball now that she had distanced herself from the Diamonds so greatly. On top of that, she couldn't tell her Gems that the force they were all fighting was secretly her the whole time.
She never really mades up for these sins. None of her actions help Bismuth, Spinel or Volleyball. Her secrets leads to countless corrupted Gems, the radicalization of Gems like Jasper, and the loss of her human zoo in space.
What she does do is try to move forward as best she can. By the time she meets Greg, while still struggling with human behavior, she's actively trying to learn. She longs to change and evolve and be a good person, but she's also so deeply ashamed of her past mistakes she can't even tell Greg everything, even though she loves him. No one seems to dislike Pink Diamond more than Pink Diamond. Much like her son does later on in Steven Universe Future, Rose chooses to deal with the worst aspects of herself by hiding them and working to help others, even if that in turn causes more pain.