1. Nam doha is the og killer, this can be inferred by the fact that he kills off the villager before the events of the game start. He is also an npc so he can't leave the game like Bada can (this is the important part)
2. The current killer is basically just a hollow copy of him with no memories and no way to distinguish Nam doha from the rest of the group (which is why he dies as an npc before)
3. The villager is a playable character who has been stuck in the game for much longer than Bada, he was killed by Nam doha for trying to warn the group and shows up again at the tunnel after failing to find an exit
4. Nam doha is using the extra cassette tapes to hinder Bada's progress, as soon as Bada is about to escape he is going to use the two cassette tapes to revert their progress back to when they first started working together. (I think Bada mentioned that extra cassette tapes can be used to override other ones)