Maybe by starting a nuclear war? Anything more lethal than that?
Rev 9
The Sixth Trumpet Brings the Second Terror
13Then the sixth angel blew his trumpet, and I heard a voice speaking from the four horns of the gold altar that stands in the presence of God.14And the voice said to the sixth angel who held the trumpet, “Release the four angels who are bound at the great Euphrates River.” 15Then the four angels who had been prepared for this hour and day and month and year were turned loose to kill one-third of all the people on earth. 16I heard the size of their army, which was 200 million mounted troops.
17And in my vision, I saw the horses and the riders sitting on them. The riders wore armor that was fiery red and dark blue and yellow. The horses had heads like lions, and fire and smoke and burning sulfur billowed from their mouths. 18One-third of all the people on earth were killed by these three plagues—by the fire and smoke and burning sulfur that came from the mouths of the horses.