Original quote by french biologist Jean Rostand (1894 - 1977):
 "Kill one man, and you are a murderer. Kill millions of men, and you are a conqueror.  Kill them all, and you are a god.”
 // Thoughts of a Biologist (1939)



And the Megadeth version:
 "And when you kill a man, you’re a murderer
 Kill many, and you’re a conqueror
 Kill them all … Ooh … Oh you’re a God!”

 // Captive Honour, from Countdown to Extinction (1992)