So my speculation was right, Yoo Ri nd Suho had an affair. And the neighbour may be knew about this. That day Yoo Ri was the one who left the gate open to fled and Hee Jae died because of her. All this was recorded in the CCTV and that party leader showed this footage to Suho to keep him quite.
I think this whole mess is only Non's fiction and nothing else, cause he is a writer and no one gave him credit for his writing. So he made this whole story, and everything what is going is only his thinking. He hates everyone his family his friends and his lover( because Phee told him to go and die) so he made this story as a revenge story. That's why he is the victim and also the main character of his story. So, I think in the end no will actually die and nothing will happen to anyone, it's just Non's imagination.