I think it is the second lead Seok Jae Beom that killed Park Won Seok! Think about it! He lost his memory, and they were so close to one another, i think once he pushed Park Won Seok, he had a trauma attack, causing him to lose his memory. Gi Oh-Seong was the witness, he locked everything up to protect his best friend. How did i get this conclusion, well we all know EVERYONE loves plot twist, and this is my plot twist.