
What they are doing to him--cold and calculating--is worse than what Masato did out of pain. I think Naru is doing it to intentionally force Masato to own up and change....but he's hurting both Asahi and Masato and it seems really shitty all around.
What if Masato harmed himself from a second broken heart?

I agree with you all. Kousei’s reason is selfish revenge, Asahi’s too passive (as proven by his last relationship) to make things right, but Naru doesn’t have one single good reason to fan the flames in this situation. Either his character isn’t written/developed well enough and there’s a hidden back story we haven’t seen, or he’s meant to be a natural asshole.
Naru is acting like a complete psycho right now with his lack of emotions towards everyone who’s affected by this elaborate scheme. He’s like the mastermind and they’re all his pawns in this game of love. That’s a shit way of teaching Masato a lesson about love because no real friend would do that. I can’t imagine why Naru would do this to Masato unless he’s been in love with him the entire time and he just wants Masato to suffer and run back into his arms.