Herbert is dead.
Maybe the real Walter is dead and Herbet took his place. :(
To get back to your Therory: Reminds me of Persona 5 Royal Sumire Yoshizawa. Walter's Action in his Childhood cause his Twinbrother death, deleloping sort of schizophrenia or divided personality as defense mechanism. This would be the easiest explanation for Walter being joyful, clumsy, forgetful most of the time just to switch into boiled up super aggression + don't-care-about-people-and-consequenzes when being triggerd by a bully or Mark's careless jokes.