The officer can't use that nail because it was taken illegally. Evidence needs to be taken with valid methods or else is invalided on court. It's implied Sangwoo and he know each other for a very long time, before the "murder-suicide" and maybe Sangwoo knows something about Kwak. They talk about making an agreement of not crossing each others lives never again, why a detective would do a deal of this kind with a victim? If Sangwoo has something on him, he put the nail to test if the deal still on and Kwak could have seen the nail as chance be free. Then he could plant it as an evidence latter after getting an warrant to search the house and blame Sangwoo for the girl's murder and also for his mother's (since Seungbae suspected him in the past)










having a blast reading the comments