Alright So I wasn't gonna say anything before but I guess will just leave it here...

I read so many theories and speculations and It all collectively make sense, also could be I am sleepy as well hehe. Anyways so what They all had some roles in developing a virtual reality game. And during their initial trial phase they got stuck inside the game. And Bada being Seongchan's cousin knew they were developing the game and was also their friend since now they are nowhere to be found could be possibly stuck within the game. So he learns about it and goes to help everyone out but gets stuck himself too. And by that time the red hair guy (forgot his name) and other twos already became npcs and only Doha and Seongchan were still playing and at that time Bada joined them and helped them and doha became obsessed with him and Seongchan who could have felt complicated emotions and got jealous and took out Bada's cassette and that made his character npc too and Doha then could have possibly taken both his and Seongchan's cassette out and killed both of them but somehow Seongchan saved his data in some other cassette too and started playing with Bada and Red hair guy again and at some point when then must have been close to some alternative ending some technical error or glitch occurred which brought back Doha's saved data or someone by mistake exchanged the cassette (remember it's said Seongchan removed that red hair's cassette and kept behaving as npc all while bada played alone) at that point he must have found out some issues occurred and that Doha was back in game and has his data saved up and to save his pathetic ass he acted as npc and at some point Doha teamed up with Bada and now that story is what he are reading and when he found out about Seongchan's npc roleplay he goes on to killing him brutally later on and finally removes his cassette as a revenge and that's why doesn't tell Bada anything about it because there is a huge bad blood behind everything. And since Doha feels betrayed by everyone who initially started on the trial game phase and who all gave up and became npcs (kinda betrayed him). That's why he doesn't care and just want to be with Bada who came to save everyone's ass and has been playing nonstop even with some of his memories or data missing which also includes their own time together. Plus there can also be the possibility that when Doha, Bada and Seongchan reached the alternative ending it probably had a criteria that only One can win and come back to reality and that's how them betraying each other began all while Bada being the most naive and innocent person who just wanted to help everyone. So now Doha who only knows that alternative ending is trying to keep Bada with himself as long as possible cause once they reach the ending he will have no choice but to choose to let Bada go back to their reality alone.