
Koogi was rushed by Lezhin to finish Killing stalking.
Remember when she said that there will be more than 2 seasons?, and also as much as I cried for the ending it was kinda rushed, everything was in favor for Seungbae.
Also if Sangwoo was a criminal in the hospital why tf there were no cops around his room?. There are so many things without answer: Why detective Lee took all of those samples of Bum?, what was the reason of the panel of Sangwoo leaving a piece of apple when they were burying the body, what triggered the first murder of Sangwoo?.

She wasn't rushed. She released three seasons (as planned) and had a lot of filler material in season 3. She would've had the opportunity to explain all of it, or portray things in a more realistic way if she had wanted to.
There are often rumors that authors were rushed when a Lezhin series ends, especially when there are still many unfinished storylines (or when the end isn't very satisfying for the fans) but they're never true.
At best the creator can't write another season because the main story is already finished and Lezhin has no interest in extending the series but that's it.
Even significantly less popular authors aren't rushed to finish their stories, even when their comic ends up being longer than planned.
If she had needed more chapters to finish the story it would've been possible for her to write them.

I agree with you but I think she simply abandoned those storylines. I'm convinced that she changed a lot of the story in season 3 to please the fans with a more "romantic" development (the way KS is written in season 3 is different from the other two seasons, she made many mistakes which didn't happen before etc.) and that made it harder for her to tie everything together.
However, there's one thing we always tend to overlook when talking about webtoons and that is the age and experience of many authors.
Koogi is a young author who has no experience in writing a long running series. This is, as far as I know, her first serialized webcomic. If she was a manga author she probably would've started with one shots and short series that are finished within a volume. But she started with a series that is 66 chapters long and on top of that has a dark, overarching storyline.
Bringing a story like this to a satisfying conclusion would be difficult even for an experienced author. And I think it probably was a little bit too much for Koogi.
Just look at the early works of your favorite, more experienced authors. Most of them won't be masterpieces and will have flaws that don't appear in later works. That's normal and I think we should keep this in mind when we read a webcomic from a young author.
Pls don’t be like easy target (⌒▽⌒)