An extreme differential between the emotional valence of having been granted one's heart's desire and having fallen to despair is essential for energy efficiency.

"That's bullshit. You're, what, you're—humans are unique for having emotions? What the fuck does that mean? Why—

"—soul gems. What's, they become grief seeds. They, you, what does despair have to do with the way gems get dark?"

It is both a cause and an effect. Eventually a magical person will fall into a mood that depletes their magic, or run low enough on magic that it deteriorates their mood. Then the conversion can take place.

"...holy shit. That's fucked up. If I go low on magic I'm mind controlled into being—no that makes total sense actually, fuck, how'd I never notice—"

"—I trusted you. We trusted you to be on our side. How—is that—you're—"

"—this isn't the behaviour of a species humanity can be at peace with, you know. This is the kind of thing that would mean war. Maybe you don't care, maybe you think you could take us, maybe you're right. But by choosing to do this you have marked humanity as your enemy."

Magical shortcuts are what has brought humanity to where it is now. Human beings were using stone tools for millions of years before inventing cooking, do you imagine it took no magic to make the leap? We have been with you at every stage of development. We have been behind every stage of development. It is really a beneficial trade if you think about it.

"Oh that's pure bullshit you sanctimonious self-important asshole. The account of the development of human technology does not have discontinuities like that other than the invention of agriculture and the industrial revolution and, sure, if you wanna say some magical person came up with those, fine, whatever, we would've gotten that eventually. The timeline of human progress does not match a series of discrete individual heroes. But whatever, do you have any parting shots before I erase your memories?"

Parting shots are an emotional behavior.

"Fine, anything you want to tell me that you think I would benefit from knowing or our relationship would be furthered by even though you won't remember it?"

No.

"Alright. Goodbye, asshole."

Rewinding fractions of a second to just skip back to right before he pauses time is also exceedingly cheap, so he redoes that conversation in stopped time a few times to see which parts of it are path-dependent. He never said he wasn't going to dictionary crack Kyūbey if it cooperated, after all. But it does seem like it was telling the truth, or at least that it's very consistent with which lies it tells. Can never discount that possibility. He is never taking anything this creepy little creature says at face value ever again.

Okay. Now time for his delayed emotional reaction.

...he's not having much of one. It seems like now that it's no longer fresh and he's already vented his anger on the weird alien he's just feeling numb. Although, well, who knows, he's the king of denial and emotional suppression, it might come hitting him later.

Actually, scratch that, it's absolutely going to hit him later when he finally sees Haru again. He will not look at Haru in his bed, he doesn't want to break down crying.

So. He transforms again, roof hops to Haru's place, and drops Haru's notes to self on Haru's desk.

And alongside those he adds a note, in English:

I don't actually know the contents of these notes, but it is VERY IMPORTANT that no matter what they say you DON'T USE TELEPATHY.

DO NOT TRY TO USE TELEPATHY NO MATTER WHAT, IF POSSIBLE DON'T EVEN THINK ABOUT IT

I can provide other evidence of magic than that. Text my number whenever you want, but absolutely under no circumstances use telepathy.

He adds his number to the bottom of the note, and then goes to sit on the roof.

why??? why not?????

Oh he is, don't worry about it.

The creature that mediates it is untrustworthy for reasons that only became apparent after the last thing you wrote

That was VERY shortly before the fight! what did he do?

I don't actually know what your notes have covered so far but

it turns out witches are actually magical people who fell into despair and/or ran out of magic

if either happens the other does too

and it knew and the process through which this happens generates energy that they harvest

it's the whole point of the endeavour

I told Ren I needed to skip school and she's just left. You can come in.

Knock knock.

Haru opens the door. He blinks assessingly at Yutaka, and stands aside to let him in.