The Walrus is a symbol of death in England. Paul sings that line, to state that he is the one that died.
If you look at the Abbey Road album cover, it shows Paul as the dead one as well. Harrison is in white, in front, representing God. Lennon comes next, in a suit as the priest. Then comes Paul, barefoot, (you were buried without any shoes). Next comes Ringo in jeans, the gravedigger.
"No I'M the walrus you dirty bastard"