>>103974823>cheated on her husband>abandoned her kingdom>joined up with her kingdom's ancestral enemies
Eclipsa's fatal flaw - and one the show largely ignored - is that she never wanted to put in any effort. Whenever she's faced with a problem, she just magics it away, or brushes it off. She's a dark mirror of Star at her least responsible, and for all of Star's angst over how she and her mother aren't "real" royalty, they both put a huge amount of care and work into protecting and improving Mewni.
Eclipsa could have been doing exactly what Star was doing for monsters, all those years ago. She didn't, because she didn't give enough of a shit; she wanted to fuck one particular monster, so she abandoned her kingdom to do it. What's the problem, right? The result being that her daughter was raised into a psychopath, her name was erased from history, and Mewni-monster relations were kept in the gutter for generations.
Eclipsa did nothing wrong. She did nothing right, either. By and large, she did nothing at all, and that lazy entitlement is precisely the problem.