Marethyu (the name) seems to come down from Sanskrit "Mrtyu" of several thousand years ago. It's cognate with the Roman/Latin words "Mors" and "Mortis", both of which mean "Mortal" - like a "mortal wound" : one which will cause death.
Quite a few cultures and their mythologies have words for death (the process) and Death (the personification of the process):
Hindu - Mara (probably directly from Sanskrit)
Canaanite - Mot
Celtic - Morrigan (where have we heard of HER before?)
Latvian - Māra
Slavic - Morana
Josh, in becoming Marethyu, has become that which all humani fear unless they have accepted death as being inevitible - "The next great adventure", to quote Albus Dumbledore. He's not a monster ravening for the kill. He may be looking for the owner of a small, copper-sheathed book. Not to take the book, but to give its owner certain ... instruction ... on how to use it.