So...

Same pls smn explains

im confused too

Doha was sus that cousin guy knew about this being a game. Point on him not giving Bada any help when he was pointed up as a sus person in the previous run, and how he seems to be able to see Doha putting the cassette tape on the thing while he don't have his glasses plus it's dark there. So Doha be sus of him.

Oh, I see

I don't know if the cousin knows this is a game.
Explicitly Doha mentioned that he's just selfish and a liar. So,
1. Maybe their friend Yong Taek died because the cousin pushed him onto the killer to save himself. When Doha says, "It's a good thing you weren't killed" it's a highlighted speech bubble. It could be a clue the author wants us to focus on. Like, the cousin is a liar.
2. Similarly, he didn't defend Bada when he was labelled as a dangerous person. So, the cousin is a selfish person.

Or it could also be that Doha said the cousin's a liar because he's lying about how poor his vision is exactly.