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)