When we're first introduced to Hyun, he's portrayed as being a complete ingrate, even causing a scene at his family's funeral. However, further revelation of his backstory reveals that his family turned a blind eye to his bullying and told him to accept the abuse he received, as the bully was the son of his father's employer, and they feared that Hyun's father might lose his job if he spoke up. And to further betray Hyun's trust, he overheard conversations from both his parents and sister blaming him for his own abuse and calling him a burden. He only became a Jerkass when he realized that no one really loved him and his efforts at standing up for himself were unappreciated.
Sang-wook/Ui-myeong seems oddly mournful about seeing Sang-won's body, an odd reaction for someone he's never met. The end of Season 2 reveals that he is Sang-won.
In the last episode the apartment building crumbles and apparently collapses, probably with at least one monster still inside it since no one ever knew how many there actually were. (The characters' trip to the car park beneath the building showed several monsters were lurking down there, and they may not all have been killed by the flamethrower, so there could be any number of monsters in the building.) Fire is the best way to kill monsters; they can survive being stabbed, falling from near the top of the building, and other injuries that could be fatal. So any monsters in the building probably survived its collapse and are now on the loose... and they aren't all as benevolent as Myeong-ja and the slime monster were...
Most monsters had innocent, or at least vague, desires (such as how The Beefcake was presumably some guy whose desire was "to be strong") and still transformed into feral, deformed creatures. With that in mind, one can only assume how Gapsu Seo's monster would have been like, give that his desire was likely something to the effect of "to rape women".
In Shotgun Boy, Gyuhwan desires to kill all his classmates, not only his bullies, because they turned a blind eye for the issue. Except that is not true, there was at least one person who did try to stand up for him, Shinyeong, and got cast out as a result. Is not pushing it to say there were others who tried to take action, but to no avail. If he went along with his revenge, Gyuhwan could very well have killed people who did really care about him. Not to mention, it's downright said that some bystanders did not took action out of fear, while Gyuhwan himself was terrified of his bullies and could not even stand up to them, meaning Gyuhwan meant harm to people not much worse than himself.
