So the subway killer is the son of the caretaker of the mother of Ji seok, and he is also a part of the grasshopper. I think he met Yoo Jin at the hospital and fell in love with her, or maybe start having an obsession for her. And start following her which is why he abducted her in the train because how would he have know where she was. I think Yoo Ryung keeps making bad decision because since she was little she probably had to take care of her sister even before the death of grandmother. So she thinks only her and only by herself she can fix everything and catch the killer. It's probably hard for her to lean on other people because she's not used to it.
https://youtu.be/6y8axpwlOWE its the link to the first episode the other are on the same channel and if you can't find them (subtitle) click on the "parameter button" and "subtitles" if they still aren't there u can go on "add subtitles" then u follow the indications and see if the episode is subbed u can watch the episode and correct if there is mistake. I hope what I say make sense I'm not a native english speaker sorry.
I hope what I say make sense I'm not a native english speaker sorry.