She gave up everything for other gems and then later gave up what was left for humans. If she didn't want Pearl to be free she wouldn't have freed her. Telling her to never speak of that secret was her way of making sure her friends stayed happy, which was her goal. Knowing Pearl would say nothing versus knowing Pearl would say nothing is the same thing.
She forgot Spinel, there is nothing to suggest she intended to leave her there forever. It's not great but it is hardly evidence of cruelty.
Evil implies intent, it is canon she did not intend to leave her. Negligence at best which is hardly something only she can be blamed for considering others knew about Spinel and also forgot her, including Pearl and Blue Diamond.
I'll just come out and say that Spinel is an awkward reveal when everything is said and done. A character like Blue who maintained the Zoo for her sister all those years never once bothering to visit Pinks personal Garden just seems cheap and stupid to me. It reeks of being an afterthought on a meta level.
Bismuth is a sad case of war, just like all the gems who where shattered or corrupted. It wasn't Pinks intent to harm anyone. The Diamonds told her so long as she was ruler of earth the colony would continue so she removed herself from it. I do not think it is unreasonable for Pink to think they wouldn't care considering how they ignored her every plea and would more often then not lock her up for trying to do things her way.>>110991317
The level at which gems can break their "purpose" is impossible to determine. It's clear Spinels programming made it very hard for her to deal with being angry but she still managed it in bursts.