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Nutcracker later referred to the sugar dolls Marie sacrificed to save him from the Mouse King as his own subjects. This means they were alive, which implies that they were Eaten Alive. Knowing what a monster the Mouse King was, it's safe to assume it wasn't swift and merciful, either.
This being a ETA Hoffman story there are lots to be found, Plenty of his stories feature harsh events but they aren't really dwelt upon, they just happen and the story moves on. It's up to you to think about just how gruesome some of which are
The Mouse King and his mice are clearly eating clawing at the sweet subjects and the dolls in the battle. So yes they were left to die in agony
Also in the back story, mouse traps aren't really nice ways to die, and regardless of if the nobles were all at once slain or if their mother had to watch them one by one die...still horrifying
Then let's talk about the nut. The other would-be-princes..if they failed at be able to crack a nut and they had to summon a dentist, they pretty much means their teeth and/or jaws were pretty gnashed up.
And then what about the off-screen battle. the Nutcracker's own words (varying on translation) are usually akin to "he's bathed/swimming in his own blood."...yeah again disturbing