Luna's darkness started to resurface after she lost all her people and after she witnessed Clarke & co. excusing everything they do with the greater good. Luna thought that people shouldn't be saved no matter what, that there are lines that shouldn't be crossed, that not everything can be excused. Granted the transition could have been smoother but her POV made sense after what she witnessed and after she realized that the sky people would stop at nothing.

It's a bit weird, right? Maybe it's just lazy writing, but there must have been a way that would have stopped Luna from taking the bunker while also saving more than just her own people. If Clarke really cares about saving mankind, then she should know that a group of only a few hundred Skaikru doesn't give them the best chance of surviving after the death wave passes. They need as many people as they can sustainably fit inside the bunker to avoid eventual extinction. And you know that Clarke thinks about these things because of the reasoning she used to make her list earlier this season. If she can say that it's more important to save young, healthy women who can reproduce, then she knows that they need way more people.

I just know that if Luna had won and taken the bunker everyone would be like; "why didn't they just take the bunker earlier when they were busy fighting, how stupid!?"

I'm not excusing Clarke but neither am I condemning her.