But that's not true either. If it were, there would be no eredar lords, no nathrezim, no succubi or imps, etc. All these members of the Legion come from worlds of their own, so they don't kill just to kill. They seek to unravel the structured order.
For example, the Legion apparently don't really have much interest in doing anything to Durotar. It is as it is, not what the Titans made it. The only interest they had in the planet was the draenei and orcs as a tool to use against Azeroth.
We can't be totally sure, but thus far what we know of Sargeras is he's focused on Titan touched planets. If the planet is in its original, natural and thus chaotic state, he doesn't really care.
Hell, as much chaos as the mortals of Azeroth cause, one wonders if we do Sargeras proud more than the Legion. If not for that pesky Well of Eternity's power.