I'm sorry cuz there's no potential for LITC to have a bad ending, Just because an author made a psychological horror that is so well written, doesn't mean it that, that element will sweep to all of her works her first manga was literally fluffy smut and there was so horror element to it whatsoever, you aren't considering that writers can bend their own creative writing.
And if LITC were to end on a bad note, it will be bad because all of that build up of character and story only for them to end up absolutely alone when there's no slight hint that the characters will have a tragic end is so downgrading. Paskim herself said indirectly in an interview that LITC is a story of two characters healing from their past. To end this story with a bad ending will lose the whole purpose why LITC was created.
Imma say it again cuz I wanna get my main point straight, just because an author made a super scary manwha doesn't mean it will be the same as all their works. I'll give you more examples of authors that differ genre in their works yet all is beautifully written and was made well-intented.
Skylar and cirrus will have good ending, this author literally implied they'll have a both nice ending for themselves because paskim cares about her characters. So don't be discouraged everyone!