...you think Haruka's just trying to test Kou, somehow? I imagine even he couldn't be dumb enough to not understand that Nazuna would probably just step out into sunlight if Kou died. She'd probably murder him in retaliation before she commits suicide, too.
Kou's not thinking clear, either. Haruka notices that Kou can't confidently answer as to how long Nazuna's gone without blood, and Kou also tried to rationalize away the problem of Nazuna's increasing weakness due to not drinking blood by saying that it'd take a decade for a vampire to starve to death, as if he intended to wait that long. Of course, he's not intending to kill her, but he's not intending to wait that long, either... he's not intending
anything beyond not going to school, in fact, but he's also not intending to risk letting Nazuna attempt to vamp him, so it calls into question why he resolved to not go back to school. Without him jumping to considering having her drink blood donations, it seems as if
he also doesn't want her to drink anybody else's blood. Not because he wants her to die, of course, but because that's "their thing".
I think more than anything, Haruka is venting his frustration over Kou's indecision and the harm that Nazuna's incurring in the meantime.
That panel of Haruka's twisted-to-fuck hand and broken finger with the nail ripped out.
Ow.
Reminder: despite Viz'
and Tonikaku's separate translations of chapter 1,
when Kou was confronted by Sacchan, the internal monologue in the raws is outright "Shut up, ugly. I'll kill you."
As the kids would say, Kou's got that dog in him. If he didn't look up to Mahiru so much, he probably would have been a classic delinquent instead of an honor student-- hell, even his way of vampire shifting involves piercing his ear like a delinquent.
I like that subtlety.