with only 2 chapters to really brainstorm, anything's possible.
It's just doing something like having the killer be a 3rd party makes the game redundant(as the situation is supposed to reflect the game, if they can't guess the person then it's not really the rabbit game) and would, at least to me personal, feel like a badly written cop-out.
I had forgotten to assume the next person to die... I'd say Haruka. Only think that way through the process of elimination...
Protagonist is well, almost immune.
The story has already determined that the protagonist has promised to 'protect Mitsuki' no matter what. Which usually grants some protection from plot elements. + my theory/bet.
Hajime has proven to be the brains which will relay all the discoveries to the reader as a plot element. He's also been way too helpful already. "could just be an act", yea well, he could have played stupid and gotten the same results.
Eiji would normally be seen as the prime suspect as he was the only one(aside from the protagonist who was knocked out in the washroom) who wasn't in the karaoke room when shit seemingly happened and he was the only one who harbored angst against Rie. It's the 'too obvious' rule. He'll draw attention away from the actual killer for the readers..
Haruka currently hasn't shown a clear role and her bar code has already been used once(they'll probably have more than one use but..). There's nothing I could see to keep her in the game.