I swear, I advise most people here to read the novels, there are quite a few bad info being thrown around and it's because the manga is not giving people enough context.
In the Novels all the reasoning behind the demon queen actions was properly explained, she is being idealistic yes, but it's the only way to try and reason with the other races, with their race being the villains of the last war, she knows full well the problems with her plans but she did make proper reserve plans to deal with the situation, everything that you guys have been discussing in the comment sections have already been addressed in the novels with the Cruel making their own observations about how appearently naive she was being, yet she managed to convince them in the end because she thought things through.
She's not the one commiting mistakes here. Same as people who have been blaming the heroes for this and that. Even in the novels the heroes never touched a single civilian, they were stopped by Ornoth before anything could happen, the heroes didn't even know they were heading to a burstling scity when they were ordered to advance, leaving them as shocked as the civilians under attack when they saw what was happening.
Also don't think you know everything at play yet, be prepared for the next chapters.