Timing is everything. I don't advocate for the slap, though it's more culturally normal in Japan than in the West, but... He slapped him at the phrase, "are you going to *buy* me" - as in property. He lost it at Ren considering himself property. Understandable outrage if you love someone. Then he stated he was already in too deep, but Ren needed to go now (likely because Haruto needed to cool off, since he'd just, you know, hit him.)
Timing is everything. I don't advocate for the slap, though it's more culturally normal in Japan than in the West, but... He slapped him at the phrase, "are you going to *buy* me" - as in property. He lost it at Ren considering himself property. Understandable outrage if you love someone. Then he stated he was already in too deep, but Ren needed to go now (likely because Haruto needed to cool off, since he'd just, you know, hit him.)