I agree that it was a bad plan, but I think what Naoko might have been trying to do is set her up for us. I’d like to think that Naoko isn’t writing her own main character “out of character” and instead, was planning the story around her development.
Usagi is weak and controlled by strong emotions at the beginning of the story and this is really clear example of that weakness. We see her sacrifice others and herself here for her own self interest, but later she develops into the girl who is sacrificing herself for everyone (literally everyone, ever). It’s more impressive to see where she’s come from in terms of her own personal weaknesses when the whole story is taken in context. ^_^
I could buy that if Usagi was portrayed in any way as guilty for her terrible choice of action, but this is not the case.
Luna blames herself for having forced Usagi to fight Mamoru and for failing to protect her, the inners are fully focused on rescuing her, and Usagi herself shows absolutely no regret or even the slightest acknowledgment of the consequences of what she did. In fact, the only reaction we get out of her concerning it is “oh… the pocket watch took some of the blow. oh welp.” She doesn’t blame herself for Queen Metalia having grown this powerful or even when she finds out the inners had sacrificed their lives for her, and neither does anyone else. Naoko portrays Usagi as nothing but a victim during all of this, which is why I highly doubt she was intentionally trying to make Usagi come across as this horrible and selfish. If she had, she would have thrown in some kind of blame on Usagi, either from Usagi herself or others around her. This is all just bad writing.