Break the horn quick! Honestly, i can get his emotions making him react quickly (and that he’s an oni so he doesn’t particularly mind killing humans) but still irks me he killed that guy without a second thought
As @Red225 called it last chapter Imperial Oni was just testing her resolve (still don't trust him tho).
Hawson proving himself to yet again be the brains of the group
I think that the Eye reacts to Oni in general. Imperial Oni seems to have the ability/magic to "take" Oni power away from Oni and basically make them human. My guess is that he either is some divine level being posing as an Oni a la Frau or he "took" his power away when she was about to attack, this making him human at which point the eye stops working.