I don't think Steven Universe Future was trying to give Pink Diamond a "reverse backstory"
The way Steven Universe Future treated Pink Diamond was a bit... odd. Nowadays I see fans insist that anyone who hated her "misunderstood" her character and SUF showed all her worst characteristics to show how much she's grown as a person but personally? I never saw it that way?
If there was a balance between showing the harm she caused as well as the good, I would understand it more, hell I think thats what Steven Universe (main show) was going for. Yes she lied to Garnet, but she truly did see the beauty in their fusion. She manipulated Pearl but she thought she was doing the right thing. But SUF treats her incredibly poorly. Despite this incredibly emphasis on how abuse and indoctrination can shape people, PD's actions are given this feeling of... i dont know... maliciousness? It's not like she randomly turned evil but it was reaching a point where it felt like more and more things were getting blamed on her.
And my only guess was for two reasons. 1) because Steven needed a push for his mental breakdown and 2) I think the writers were responding to fan backlash.
Because admittedly, I didn't really love how the main show handled her either. The show kind of had this "she made mistakes but she meant well" mentality but the terrible things she was doing just kept increasing. like it went from "oh she was a bit dismissive of pearl's feelings" and "oh she didn't take greg's romantic affections and fears very seriously sometimes" to "oh she was pearl's master and forced her to do something horrifically traumatizing and keep her quiet about it (but Rebecca insists that PD viewed her as an equal?)" and "she allowed a war to happen that shattered gems on both sides and lied to many of her trusted allies who literally died for her", but the show still tried to push this "oh she did bad things but she meant well" mindset with her writing that sat weirdly with a lot of people. I think my personal breaking (and what pissed off a lot of fans) was the Bismuth episode which was pretty much the first time PD did something unquestionably horrible and the show just... sidesteps it hard. Bismuth comes back after multiple seasons and she just laughs it off like PD didn't ruin her life for no reason. By the time it was revealed that Rose was PD, i think a lot of fans just turned on her. We wanted the characters to stop making excuses for actions or ignore what she did.
SU future (and the movie) felt like it was trying to do that, but then it started to feel egregious. rather than really exploring what she did in the main show, they introduced more and more awful things that she's done and there was really nothing to be said about it other than "damn that sucks. poor steven". I don't think this was the show trying to show us PD's flaws to see how much she's grown, it honestly felt like they were trying to use her past to paint her in a very different, negative light.