@sgathach random, but I love your name though I can never pronounce it properly.
I believe what's happening is: MC originally liked nakaya because he resembled "Hotaru" the character. But actually, Nakaya purposely acted that way because he was already aware of the obsession, so his friend wants him to stop lying and confess that it was all a scheme. Now MC really has fallen in love, but Nakaya thinks it's only because he was acting the part - so it's half guilty conscience half fear that admission will lose his love
Saying Hotaru will die is probably his declaration that he'll stop the act