The ultimate theme of season 3 turned out to be taking responsibility. I am not a huge fan of big season finale's necessarily, because I feel they often create a scenario where the show constantly has to top itself and yet, "Crazy Ex-Girlfriend" has managed to do that three times in ways that are both unexpected and predictable. Anyone who watched the first episode would not have been terribly surprised to watch Rebecca stand up for herself in one relationship only to immediately turn around and make the choice to hook up with Josh mere seconds after they each left a relationship, and then just assume that this meant all the things were perfect.
And it wasn't a surprise in retrospect that Josh would not marry her, would leave her at the altar, and vowing revenge was wonderful and right and also totally not the healthy choice.
Rebecca realized that she felt guilty for having been absolved by her friends for the smaller transgressions, and decided in typical Rebecca fashion to go all in and confess all the things, leading to understandable hurt on the part of Paula, Nathan, and Josh. Normally I would say the big gesture was Rebecca hoping to make herself feel better, and I think there was a little of that. Confession is good for the soul, but there are some things that you confess to feel better about not having to keep the secret, not because anyone really needs to know what had happened. And of course, there is always that hope that the truest of friends will hear the whole list and still forgive you. Ultimately, I think it was partly Rebecca expecting to lose everyone, because she realized that she really had crossed boundaries (and legalities) and wanted to be punished because she doesn't really believe she deserves good things.
So post confession, having been manipulated by Trent to show up at Nathan's housewarming party with his new girlfriend, because Trent threatened Nathan's life, she shoves Trent off a balcony and then ends up in a scenario where, as Nathan neatly sums it up, it looks like she showed up uninvited to one ex's party in order to attempt to kill another ex. So, he suggests she make use of her new diagnosis and plead insanity. But Rebecca, after singing an awesome duet with Nathan about how nothing is their fault, ultimately realizes that that's not the road she wants to take. So she pleads guilty.
Season 4 is apparently the last season, so I fully expect that the creators are taking us on a final amazing journey. I'm so grateful for this show.