After rewatching, it all depends...

...it all depends on can we believe the first encounter we are shown of Do Joon and the victim as an objective view. We are shown that the rock was thrown AFTER Dojoon had already turned and had taken several steps away from the abandoned building and zero dialogue between Do Joon and the victim. (Other than his taunts at her, but no dialogue once she enters the building.)

My first viewing, I was pretty certain Do Joon, without malice, had killed her. Now, I'm not sure at all if the junk man's story was true.

EDITED
Now that I've reached the scene with the junk man's story, I've changed my mind again and think Da Joon definitely did it. But imdb won't let me delete my post. Must be cause I'm on my cell phone. *sigh*