I think it might be along the lines of flowey realizing he's about to be the guy killed by the rock.

Something like that.

Unable to feel emotions due to being soulless, and trying to muster at least hate from being a photosynthetizing little shit, but all it takes is a little shit dropping a non sequitur* to make him accurately emulate fear.

• It's still a non sequitur if the character's special/supernatural circumstances allow them to actually perform it? In this case it could mean anything from "backhanded threat", "Frisk/Chara's just chit-chatting but Flowey realizes he didn't think this through" or simply, literally "I'll become an astronaut, kill my crew then reload".