Complex sentence: So-Hee leaves and as Joo-Yeong keeps rambling about it by herself Eon-Joo appears behind her and is killing her when So-Hee gets back, she sees nothing but the window open.
Split into simpler sentences: So-Hee leaves and while Joo-Yeong keeps rambling about it to herself, Eon-Joo appears behind her and kills her. When So-Hee gets back, she sees nothing but the window open.