I told Kuro to back off before because just motive isn't enough.

But once you take two pieces of evidence that point toward his involvement, add how he's likely the only class member left with the physical strength to snap Thomasin's neck, and, as Kuro did point out, the strength of his motive...

Mysteries don't hinge on a single piece of crucial evidence, most of the time. That's bad- [Yasu bites her tongue.] We just, um, got lucky with Reika. It's... patterns. That's what I've learned, at least.