And with the ending of Steven Universe Future, on my first rewatch in years, I think I'm ready to say this:
I forgive Pink Diamond, she's done wrong but it's not as if she was entirely awful either, I just think that watching things through Steven's eyes makes her seem like the worst thing and I let myself get too wrapped up in that on my first viewing.

So much of Pink Diamond's actions end up being interpreted in the worst light because Steven is, obviously, the main recipient of the consequences of all her choices, good and especially bad. And it certainly does not help that, even in life, she was both secretive and hard to understand on top of a being childish to the very end.

But throughout the show we can see that Pink Diamond had an awful... childhood? (Gem Childhood?)
She was never given responsibilities or properly taught anything and whenever she acted up or did anything wrong she was stuck in a tower until she behaved, no surprise that whenever she messes up, she has the awful habit of keeping it secret.

Spinel's abandonment is barely even Pink Diamond's fault even though it's been pinned as some sort of purposeful malicious act? 6000 years is a broad enough date for Spinel to be stuck in that garden, because that's around the time the war happened! And during the war they destroyed all the teleporters that lead off planet earth so no one from homeworld could come, so it's not as if she could visit her!

Volleyball (The Pearl) was hurt in a temper tantrum while Pink was still effectively a child, and she was never given the chance to apologize or help, since White Diamond took the broken Pearl and took control of her. (And hey, what'd you know, this could've been an opportunity for an immature Pink to learn responsibility for her actions)

Bizmuth's case is definitely the most egregious, it was directly her fault and it was during a time in her life in which she'd already developed some amount of maturity, Bizmuth simply presented an option that went against her ideals, and the best she could do was lock this "problem" away and keep it secret from everyone.
Though it is notable that Rose ended up taking Bizmuth's advice and "shattered herself" (even if it didn't solve things as easy as either of them believed)

I'm not saying that an awful childhood excuses her, absolutely not, but it does make her actions and bad habits understandable to an extent. She only ever grew up when she finally found a cause to fight for, the fight for earth (and even then not perfectly), so no surprise things got complicated!

Younger me let herself get swept up too much in Steven's point of view, I kind of just let myself... think of her the same way Steven ends up thinking of her.
A flippant tyrant that did as she pleased with little regard for the thoughts and feelings of others, only wanting to satisfy her own curiosity most of the time, and as an outside observer, I thought of her as a flat boring badly written character, but this rewatch proved to me otherwise.

Addendum: I should also mention that throughout the entirety of Steven Universe many parallels are drawn between Steven and Pink Diamond in how they developed and grew up, and SU Future goes so far as to show that Steven glowing pink is the exact thing Pink Diamond went through and that it was brought forth by the same feelings she had, the exact same anger, the exact same sadness, the exact same feelings of inadequacy, someone who has closed themselves to others due to the harm they've suffered from others, to the extent that even close relationships feel alien.

The only difference is that Pink ended up taking millenia of self actualization and a cause to literally fight for her to get to the same point (or a worse point, I'd argue) that Steven reaches by the end of SU Future since he actually had a decent support network of friends and family that cared about him instead of just being the idol/figurehead of a revolution that naturally created space between you and others.