Why Bada?

But we only see him attack bada after he got his tape removed and forgot his memories. So maybe he was instructed to kill him on site to prevent him from getting them back, especially since his tape was on the npc.
What I'm more curious about is how the cousin convinced the npc? I don't think you can reason with them otherwise they'll also help during the game. And bada, who's supposedly a former npc, had to have a tape inserted to become a character, but even then that gave him free will, unlike the npc who seems more mechanical than an actual ally to the cousin.
My theory is that bada was never an npc to begin with and there's a 3rd tape which contains his original memories. Since both him and his cousin don't make sense in the context of the game, and they seem so out of place compared to the other players, I'm thinking maybe they're both the original creators of the game, but the cousin's jealous and backstabbing behavior got the best of him and he ended up messing with the settings?

Me too I've been thinking for a while that Bada never was an NPC. Thought that maybe he found an alternative ending to the game or some cheat code that eventually erased almost all his memories from the game (but with him still experiencing physical reaction born from trauma when approaching the playable map), and let him outside of the playable map. His chara design is too designed to be one of an NPC (compared to Seongchan's ally or the killer). Even his name bears meaning ("sea of blood" if I recall). Like, he's "just an NPC" that warns people in the prelude then it's bye-bye for the rest of the game, so why such details...

I also agree that he's too well designed to just be an npc. To begin with, an npc is supposed to be forgettable, but how can you forget someone like bada with his design, his clothes, hair style/color, even his skin tone and name. He's just not someone you would ever look at and think "npc". And if we're gonna talk about npc looking characters, it would be everyone else, even the ml if you will, because he is a bit too "ordinary" for someone who's a main player (not an insult, just that he doesn't stand out much in terms of appearance, the only thing that makes us, the readers, notice him is because we know he's the ml an we know how unhinged he is, otherwise if he kept acting "normal" you wouldn't think too much about him).
Also there's the fact that he assimilated a little too well to his transformation from npc to npc with a tape. I don't think any other npc would just easily slip into the role (?).
And if I remember correctly wasn't it the cousin who suggested they make bada help them? Why him specifically and not an other npc? And what made him so sure he would be fit to help them, even if they did have a suspicion that he is good at horror games, which again why would an npc even have that sort of knowledge if he's just a background character?