It's a simple tactic, that isn't actually very helpful.

The Assyrians effectively exterminated Babylon, only to eventually fall to her armies many centuries later. Even the jews mentioned something along the lines of "no one will miss you when you're gone".

You can't actually exterminate your way to peace. Maybe for a life-time, but not forever. God help you when your empire of terror falls, there won't be mercy.

That's why the Mayans... were eaten when they were finally defeated.