1. /give [your name] minecraft:commandblock 1
2. Place Command Block.
3. Set Command Block's first option button to 'Repeat'
4. Set Command Block's third option button to 'Always Active'
5. Set Command Line to say 'kill @e'
Congratulations! All life has been extinguished! Revel in your new status as the lord of a dead world, from your lovely (dead) view of your spawn point.
DISCLAIMER: Astrotrap are not responsible for any damage caused to the playability of your world, to the livelihood of your chicken horde, or to the livelihood of your chicken hoard.