Q1: the cousin got caught messing with their tape saves, and he’s also an asshole so 100% something bad would have happened if Doha didn't handle that somehow. Unclear the consequences of snapping the saves (will likely happen at some point in the story tbh) but it will definitely erase the save for a run or maybe make them forget about everything so far altogether
Q2: eliminating variables basically. That guy is dumb and doesn’t even remember everything each game. I think at one point the others turned on Bada and Doha bc of it.
Doha seems to dislike him as he noted that the cousin isn’t very close to Bada and is willing to sell him out / not speak up for him in situations where Bada is suspected of being a literal serial killer, kinda rude for family lol. Iirc Bada says that they don’t really meet together much and aren’t super close (but never implies that he knows how much the cousin dislikes him or holds him in contempt)
Also, if you look again at the chapter where the cousin takes the tapes, he rifles through Bada and Doha’s bags because he thinks they might have some sort of relationship together and also expects to find things like drugs. The way he talks about this looks to me that he either is going to tell everyone else to give Bada a hard time or blackmail him, instead of not doing any of that or being concerned about his cousin like a normal person

Can someone explain to me or their theories why Nam Doha killed Bada’s cousin on the last season? Why does he need the tape of Seongchan???