While I'm looking back at Seasons 2 and 3 today, I just want to say that it always grates on me so much when people say something along the lines of “Catra tried to destroy the world just to spite Adora.”
Because no. That's not why she pulled the lever. Or at least it's only one tiny, tiny part of a much bigger picture. To be honest I blame this on Season 2 and 3 being split the way they were, because I think if you look at them as one whole a lot of things make much more sense. With that in mind, let's look at the events that lead to Catra's decision to open the portal no matter what:
Shadow Weaver tricks Catra into helping her escape, pretending to give Catra the one thing she's always wanted from her affection and abandoning her to face Hordak's wrath.
Catra finds out that Shadow Weaver went to Bright Moon to find Adora, once again seeming to choose her ““favoured golden child”” over her. Obviously that's bullshit but to Catra that's genuinely how it appears.
Shadow Weaver shows up in the Fright Zone with members of the Rebellion and proceeds to attempt to severely torture/murder Catra with their help.
Bear in mind I'm describing this the way Catra sees it. She's of course not correct about a lot of it but to her this is what is happening.
You see the common thread in every moment which drives Catra over the edge to the place where she chooses to pull the lever? It's Shadow Weaver. Like always, it's Shadow Weaver. Adora really has almost no part in Catra's reasoning for opening the portal beyond being the person onto whom Shadow Weaver redirects Catra's bitterness and resentment.
Until she found out Shadow Weaver had chosen Adora over her again, Catra was genuinely considering never going back to the Horde and just staying in the Crimson Wastes. It was only after that discovery triggered her childhood trauma that she changed her mind and resolved to open the portal.
Now, I want to emphasise that opening the portal was still the worst possible decision Catra could have made! I'm not defending her actions in any way!! But please stop trying to make it as simple as her being pissed off at Adora. Because it was so much more than that. This was the culmination of everything Shadow Weaver had been doing to both of them their entire childhood.
When Catra pulls that lever, that one moment is a microcosm of the wedge that the abuse they suffered at Shadow Weaver's hands drove between them from the start. It's the catastrophic result of the way that Shadow Weaver's manipulation led Catra to project Shadow Weaver's belief that Adora is superior onto Adora herself, and left Adora with a hero complex which only worsened the divide.
Pulling that lever was entirely Catra's fault, and it's something for which she had to take responsibility and accept the consequences. But it really bugs me when people ignore the way that the manner in which she and Adora each internalised Shadow Weaver's abuse led to that point. They suffered equally because of Shadow Weaver when they were children, and that is the root of the core conflict of the show.