We know that the Black Moon clan attacked Crystal Tokyo immediately after Chibi Usa took her mother's crystal, resulting in massive lose of civilian, not to mention destruction of the city itself. But do you think she's culpable for what happened? Were the Black Moon clan waiting for an opportunity to strike Crystal Tokyo and when they realized it was gone they struck? Or were they going to attack anyway, and it just so happened that it coincided with Chibi Usa taking the crystal? And do you think Chibi Usa, as a child (granted, a 900 years old child) should be held accountable for what happened?

I think the fact that Neo Queen Serenity revived most of the people who presumably died during the attack sort of makes this a bit of a moot point. But if she hadn't revived everyone (IMO there's gotta be some people who were beyond help. The Black Crystal that landed in the city was massive, those unfortunate enough to have been in the immediate vicinity of ground zero would've been obliterated upon impact) should a child be held responsible for lowering the defense of the city, and be complicit in the death of those who died?

I would lean towards no...but with some reservations based on unanswered questions.

Manga/Crystal - NQS calmly walks into the storage room looking only for Small Lady.  She doesn't see her but sees the smashed display case and hurries out of the room but her concern is still Small Lady.  Mere seconds later there is a rumble and explosion immediately after that kills everyone outside of the palace in Crystal Tokyo.  So basically to me had she not been searching for Chibiusa she would not have even been near the crystal anyways. And even if the crystal had been there I still don't think she would have had enough time to react from the first warning rumble to when everyone else in the city instantly died from the first Black Crystal meteor's impact?

Where things get murkier is with what happened beyond Crystal Tokyo.  Since nothing is ever mentioned about it...  Was this an Independence Day like coordinated assault all over the Earth all at once?  Or was Crystal Tokyo targeted first and only after it had fallen did they move on to vaporizing other cities?  If it's the latter then maybe NQS could have made a stand before people outside the vicinity of the city died had Chibiusa not swiped the Silver Crystal?  In which case I could see a portion of responsibility falling on her for sure...assuming NQS could have managed to not succumb to Demande's hypno power.
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90s Anime - Well first of all in this version Chibiusa at least doesn't intentionally steal it.  Beyond that the events are much less clear.  For starters I don't believe it's even made certain that NQS even noticed the Silver Crystal was missing before she was ambushed by the Black Moon.  Also from the order of images we see it seemed like the first attack may have been on NQS (when she went outside to search for Chibiusa) with full scale attacks on the city only coming after...one does have to wonder if events could have played out differently had she not left the palace in search of Small Lady?  Would Demande have attacked the palace first?  Could the senshi then have stalled long enough for someone to retrieve the Silver Crystal for her to fend him off?  Or would the first attacks have then been on the city if she remained holed up in the palace.  With these attacks being energy weapons fired from ships instead of one big apocalyptic meteor hit would there have been time to throw up a shield to save parts of the city from being destroyed?   Honestly not nearly enough information to be able to do anything but make wild guesses here...