Because the plates of Ether were compiled by a single person, Ether, we cannot tell whether it was Ether or Moroni (or even Mosiah as the original translator) who supplied the general narrative. However, it is highly likely that at least verse 15 comes from Moroni. This part of the story describes the beginning of the secret combinations that Mormon had declared were so dangerous that they would see the end of the Nephites. The key phrase is that these were oaths “given by them of old … which had been handed down even from Cain, who was a murderer from the beginning.”
Compare that phrase to Helaman 6:29: “Yea, it is that same being who put it into the heart of Gadianton to still carry on the work of darkness, and of secret murder; and he has brought it forth from the beginning of man even down to this time.” While Mormon credits Satan, rather than Cain, both Mormon and Moroni see these oaths as being ancient, and ultimately stemming from Satan (the one who enticed Cain to murder for gain).