Who killed more than two million Jaredites?

The Lord

And now I, Moroni, proceed to give an account of those ancient inhabitants [the Jaredites] who were destroyed by the hand of the Lord upon the face of this north country. Ether 1:1

And it came to pass when Coriantumr had recovered of his wounds ... he saw that ... there had been slain two millions of mighty men, and also their wives and their children. Ether 15:1-2