
Why he's the suspect? I mean why him of all the people I just don't get that part

police can, in this situation where there is no evidence, only have suspects and then try to get something out of them while they interigate them. Shin might be just a random person that they find suspicious (aka. the real killer might still be on the loose, and we know thats true. I guess thats the point they want to show.)
Nobody would suspect a celebrity as much as they would random people. Maybe they would even avoid putting a blame on celebritiy and piyt fake blame on someone with no money to hire good lawyers or get their way out of jail.

They wouldn't consider him a suspect based on his disappearance alone, so, if they are already considering him a suspect for the murders, it is to be assumed there was some kind of evidence pointing to Jeong-Yeon found on the body. It could be something like DNA, or possibly even the knife with his fingerprints on it. Since it looks like Seo Ilmo was planning to make things appear this way from the start, he probably would have made sure to plant sufficient evidence to frame him too.
Whaaaa this is really one of my favorite villainess stories ahhhh i hope they upload more