This was one of the most well crafted point and click games I have ever played. It's not made so that you must think and click randomly for hours like others if its genre. It's made to display an absolutely beatifully written story, and to make you live in that story, if but only for an hour.
My guess is that "This one's on you." means the detective was framed. If you notice, there is no killer anywhere. Just you and a tied up woman. If you notice, as soon as you open the final door, a grey, bullet shaped object starts moving. My guess is the killer had a hidden video camera in the area, and if there is ever a sequel, you will be seen as running into a room and shooting someone as there was a gun rigged to that door. Also, the reason he was not cautious upon entry was because he was in a hurry, so he missed signs of rigging on the door and indirectly killed her.