It's interesting and fairly realistic that hiro would start to spiral down as he starts to actually heal and fall in love with airi. Just look at the way airi looks when he is fantasizing about living with her, even her naked body, she looks so beautiful, which immediately contrasts with his childhood self in his nightmare scolding him. I REALLY like how he panicked when he saw the cake. It's like he is still messed up but he has healed just enough to see how fuc*** up this situation is because he has started to actually care for airi and see that airi also is genuinely trying to be good to him. And with hindsight, I totally see why he looks so resigned when airi says she forgives him and that he is amazing to have lived this long without killing anyone else or himself.