See? This is an audience inference. You, the detached observer, assume she's only drinking his blood.
I'm arguing that it's a reasonable presumption for Kou to make, especially since Nazuna's talked up the taste of his blood to the point that she would rather look for him than suck another person's blood (consider ch.4, p.18, and ch.4, p.8-9). As the reader, I
know that Nazuna isn't drinking anybody else's blood, because she's been suffering ill effects that only Haruka's noticed so far (like frequent dozing off) as well as ill effects that nobody else knows about (like the intense craving she had at the end of chapter 188).
Speaking of which, it's not even just that he has reason to assume she's only drinking his blood-- he himself has always only wanted her to drink his blood (ch. 4, p.16), and he's never had reason to deviate from this possessiveness until now. So, it's a problem if he still has that possessiveness, but they've decided that she can't feed on him, but they haven't already decided that she (as a vampire who needs blood) find other means to feed.
You assume the boy must also have come to this conclusion, and then prepared an exact response specifically addressing this when some simp tries to kill him out of nowhere.
Not quite-- I'm just saying that Haruka has
a point in being angry with Kou for hardly thinking about something that's bad as is and will only get worse. That he hasn't even thought about it is the problem for Haruka.
Actually, now I'm thinking-- isn't Nazuna normally a day sleeper on account of being a vampire that has no business in daylight? Yet, she woke up in the morning along with everyone else in her complex in chapter 190 (ch. 190, p.2), and was falling asleep during the party hosted at night. In that chapter 190, she was also waking up near a non-blinded window, being hit with sunlight. On top of that, she even gave a line ("Today is... another beautiful day"*), as if to join the other conversations in the first couple pages of that chapter.
Apart from her blood abstinence, do you imagine there's any significance in those details? As if she's attempting to go through the motions of being humans in the vague hope that something comes out of it?
*The Japanese was 「今日も... いい朝だなーー。」, with 朝 bearing the furigana てんき even though it's naturally pronounced あさ. The translation here's correct enough, but it doesn't account for what's probably a riff on the stock small talk expression "The weather's nice". I don't know how you'd manage conveying that without a speculative TL note, though.
Yeah, that sounds like a pretty deep issue; The kind you don't identify and solve in a single month when you're a grown adult, much less a 14 year old boy with a girlfriend who's dead set on hiding it.
And I'm not going to fault Kou for not deducing that much. It's the blasé attitude he has to even the superficials of the circumstance that makes Haruka's actions somewhat sympathetic.
Initially, my thought on his course of action was, "what's the author trying to do? Not even Haruka can be so stupid as to think killing Kou will fix anything. Huh... you know, Kou's being oddly nonchalant about being told that Nazuna's not drinking blood despite it being trivially easy for her to get it even if she doesn't want to drink his."