That is the impression I've gotten from playing it myself too. Oh well, I suppose it is just not *that* kind of game! At least it seems like they know what they want to produce and are focused on achieving their goals.
As for your question, I don't really have an answer aside from an assurance that it doesn't always come from a sadistic side like the way you described it. Quite a few people enjoy such themes through a masochistic kink or two that they may have, just like some people may enjoy their sex on the rougher or more humiliating side both on the giving and the receiving end. It is a two-way street, not just some sick "I hate this guy so let's pretend this is him" kind of deal. Besides, a lot of it is purely subjective and probably can't even be explained unless you are talking to a person who shares all the required fetishes.
I just thought that a game with such an impressive damage system, real-time combat, and seeming lack of fear to portray guro themes would have been a perfect fit for a few more exotic death scenarios, but as you have accurately pointed out, so far it really does look like those are focused almost exclusively on the nest rape mechanic...
Weeell... you can also play the role of a succubus to hard raping to killing others using nympho ability everything you see (Btw - you can even become a succubus race yourself) or for example a role play a monster (Lona abomination race) kill and devouring people again and again with tentacles power to satisfy your hunger... but there most often everything comes down to the same portraits even if you work like prostitute.
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I mean it's quite a lot of work has been done on the LonaRPG in terms of portraits (There are currently 4000+ pictures in the game like changing unique interface, clothes in different poses, body condition in different poses) and Lona even has 4 personalities Normal/Tough/Timid/Nymphomaniac Lona what changes her phrases and behavior but there everything basically comes down to cruelty or rape... or you die~ + the game seems quite difficult for many people... sooo... I don't know what else to say but I advise you to try it because this is probably the only rpg game in my memory where they were able to combine good rpg element and the h-content and funny memes in one place at the same time + the game code is quite open same as portraits so you can make some edits yourself if you don't like something + there are mods and modders who can often respond if you want something special.
The problem is that I have zero interest in doing awful things to other NPCs or even player characters, but I like it when it is the other way around. I like to upgrade the character to make them strong, then find some sort of disgusting or humiliating baddie and... well, sit back, purposefully not resist, and simply enjoy watching my own character get absolutely destroyed.
And the thing with that is the slower and more unnatural, exotic, or hardcore the peril - the better. So while things Lona can do to others sound interesting from the gameplay perspective, sadly I am only ever interested in all the bad-ends and debuffs she herself can encounter...
I've looked around the wiki, stuff like slavery, body mutilation, race, or even getting sterilized sound fun, so does pregnancy and STDs. So you are right that this game has a ton of potential, even if it doesn't go too deep into non sex-themed bad-ends.