This isn't true. Two episodes ago, Nui tried to pull the same shit, at which point Ryuko told her to fuck off with that shit, and worked in perfect sync with Sengetsu, as oppose to the monster she became in their previous encounter. That's development
This is a completely different situation. There is nothing to have revenge over. The conflict is her shame about her being a life fiber. Through the entire series, she's been constantly warned that nothing good will come from being with life fibers, but she thought she was proving them wrong by creating a bond with Sengetsu. Now she learns (or is under the impression) that was all just BS and they only worked together because she is the thing she is fighting. Combine that with the intense trauma of learning who her mother was, that she tried to have her killed as a baby, that her greatest enemy is her sister, that the world is being taken over by life fibers, which she now learns she is, which makes her feel disconnected with humanity after spending the entire series forging such a strong bond with Mako and others....
No, it's wrong to say that just because she learned that she shouldn't get angry over one specific situation, she should never get angry or traumatized ever by anything. That's ridiculous. No human character is going to be like that, not even if we were 190 episodes in. This isn't a mood swing or and this isn't doing nothing to forward herself as a character. She learned, but she's never going to be perfect and it's ridiculous to expect her to be. And it doesn't even seem she is angry to me. Well, she's pissed, but that seems to be an after effect to she shame she feels at her having been seemingly disconnected to the human race. What she feels right now is alone, not angry.
I can't do anything about you not finding it interesting, but not having her react this way or a similar way, like any human would to a hugely traumatic event, would be worse.