UF Papyrus can stop his feral state of his own will, his brother can’t. Though UF Papyrus won’t kill you, he can still torture you with his annoying zero damage punches. He doesn’t know better. Is a dangerous and fearful leader at first sight.
Before UF Sans changes into feral state he touches his single gold tooth out of anger. That’s an early warning for you to run away or hide. Because this guy WILL kill you when you’re not careful enough.
And he looks always tired, squinting his eyes like that. Sometimes he seems to have troubles to recognize your face. Is a cynical meanie at first sight.