I've issue with a lot of "dark" works of media that they're unfortunately very immature and i feel that is the case for this one
i feel it happens a lot with japanese media in particular since manga is so easily available for young authors to initiate in comparison to the western comics industry (although webcomics come across the same issues), the work is meant to be very dark and somewhat disturbing but the framing and presentation are so heavy handed that it almost loses all impact
the issue here is that kaede isn't really a complete character, she has a backstory that we're learning about, but we jumped straight into her being a tortured slave and having severe ptsd, which is sad, but you can't really call that a character. So everytime she has a breakdown it just feels kind of out of nowhere, not because the author hasn't justified it on her backstory but because Kaede hasn't really earned our sympathy, she's less a person and more a very sad kicked dog, which is still sad, but isn't really that interesting.
Also, certain level of subtlety is very important when portraying things that are meant to be disturbing, not necessarily because they can be shocking but exactly the opposite, going for too much can quickly turn a situation cartoonish, it's important to temper very heavy and out there material (like slavery, torture and ptsd) with equally strong grounding elements and restraint to keep it from being so extreme that ones suspension of disbelief starts acting up and one remembers this is not really two people but two characters that an author has put in this situation to elicit an emotional response
That isn't to say that these things cant mimic real life or that extremely bad situations do occur in real life but narrative media is inherently different from real life and the way it elicits emotions is also different, a good example of this are horror movies, no one can deny that having some maniac with a knife attempting to kill us would be terrifying in real life but that doesn't mean just putting a man with a knife in a movie is going to be scary, there is extra work that needs to be done to generate the feeling of tension because first we need to become absorbed in the narrative to get to the point we are able to simulate the terror on our own, and the way you do that is you get the reader/watcher to empathize with the characters in the story, once you care about the characters you can care about what happens to them.
The manga is, again, very heavy handed and jumps directly into telling us how terribly tortured our main character is without really showing us or even giving us time to know them properly, so it constantly feels like its trying to have sad and tortured moments without having earned them
TLDR; the manga tries to have a lot of sad moments but we never really got to know or empathize with the main character or her experiences, all we know is shes a slave and she was tortured, which are bad things to happen to someone but they're not really character traits, it's a very heavy handed attempt to get us to care for a kicked dog basically, which we still could, but it isn't good writing.
and yes, i was really bored that i wrote all that