Because you may be stupid, but you're not that stupid!

[ She means Ryuko would never actually go and kill someone. She knows Ryuko can get upset, but when she's not going berserk, there's no way Ryuko would kill someone.

But that body language isn't missed by Nonon, and... it kind of hurts, making her emulate the same kind of thing. ]

Showing up at the building and just yelling out that you challenge the killer isn't going to work here like it did at Honnouji.