One thing I don’t get is tho, why so many of these manga involve rape?
When a character has some sort of character flaw due to past trauma, rape is just the go-to thing. Presumably because it’s traumatic enough that almost no matter how bad the character acts, you still feel sympathy toward them.
If you follow the history of film and other forms of drama from about the middle of the 20th century, you can see successive waves of “default” answers for the question “what is the buried trauma that makes character X so messed up?”
It starts with Death: the death of a parent or sibling, or other direct experience of death (seeing a person killed, for instance).
Then it becomes physical and/or psychological abuse, usually by a parent or someone in the home (a nanny or servant, say). The degree and nature of the abuse ramps up over time, too.
Then it becomes sexual abuse, where it has been for some time, and of course the kind and degree of that increases over time in many contexts as well.
You can see an example in the 1957 movie The Three Faces of Eve, which was the first public exploration of multiple-personality disorder (as it was then known). Like other psychiatric-therapy thrillers, it’s a kind of detective story trying to unlock the mystery (in orthodox Freudian fashion) of the primal trauma that “split” the protagonist in multiple personalities. It is eventually revealed that as a child, Eve basically freaked out at her grandmother’s funeral. From a 21st century POV, that seems almost laughably tame—“Hey, at least pony up some sexual abuse and torture, or get those multiple personalities outta here!” But there it is.