In Mormon 3, who said he'd make sure all of the Nephites would be killed?

The voice of the Lord

Bbehold the voice of the Lord came unto me [Mormon] saying:

Vengeance is mine, and I will repay; and because this people [the Nephites] repented not after I had delivered them, behold, they shall be cut off from the face of the earth. Mormon 3:14-15