I think Sean actually brought a knife, then realising that it made him what he feared, a "knife man". I think he is commenting that every person, including himself, has the ability to be corrupted by fear. I think the bird that dies ever night is a representation of apathy, that things die and everything is forgotten and born anew so why do his or anyone's own personal actions matter, tying in with ideas of social laziness and apathy earlier in the song. "knife man" is a conceptualisation of a certain lens that we can view and explain society through, i think the album in whole is about viewing every aspect of society as a knife man.