According to the book I'm reading right now (God's Monsters, Esther J. Hamori) the first reference to death reaping/gathering souls is Jerimiah 9:22 ("Say, “This is what the LORD declares: “’Dead bodies will lie like dung on the open field, like cut grain behind the reaper, with no one to gather them.’”) and in the original Hebrew, death is named Mavet. In ancient Hebrew/west Asian mythology, Mavet was an entity, often associated with swallowing/eating people who died, which also ties into the teeth on the characters scythe.