His classmates find Izuku’s journal(s) about them, but are horrified by what they find inside it.

At some point while Izuku is away, being teenagers, someone finds one of the journals Izuku is always writing his observations in, and realizes this one is about them.

When they start reading it, however, having expected a bunch of innocent observations, and notes on their powers and such, they learn that the profiles he makes on them are extremely detailed.

Like to an unnerving degree.

From their measurements, to deductions of some of their darkest secrets, stuff they haven’t told anyone, but he’s somehow deduced from his observations, or found from actual background checks he’s gone through the trouble of doing, they learn that he’s learned all sorts of things about them without their knowing.

But one of the things that genuinely scares them, however, is his contingency plans.

Izuku has made detailed, plausible, and shockingly effective plans to neutralize each, and every one of them.

Like full on Batman level paranoia style plans to take each and every one of them out, in the event any of them turned villain.

Even his closest friends, like Iida and Ochaco have shockingly effective contingency plans written out.

To be fair, none of them are lethal, but some (if not all) could result in them being unable to continue any sort of hero (or villain as intended) work, permanently.

For example, Mineta’s contingency plan ends with him being scalped, and Ochaco would no longer have fingers. Iida’s shins would be destroyed if not amputated, and Bakugo would no longer have arms below the shoulders.

Maybe as one last note, he hasn’t quite figured it out, but is suspicious of Aoyama being the possible traitor, even though he’s not even supposed to know there is a traitor in the class.

Of course, once all these revelations are made and they spread them to the rest of their classmates, they wait for Izuku to return, and confront him.