Let's do so by giving to this world the Emperor's peace: EXTERMINATUS! (Yeah, that's the end of EC's storyline, sorry for the huge spoiler - 'forgot to mention that the eldar will escape the purge thanks to their webway gates, sneaky guys eh? *Just as planned*)
A tough choice. Mon'Keigh who think themselves in power because some Dark God has fed them scraps from their table? Or the ones which worship a rotting Seer on a gilded throne? Choices, choices.
Well, I can help you to choose: you just have to destroy one of them entirely (Imperium) and the other will starve to death (Chaos); the tricky part will be to survive the second one's wrath.
I very much doubt that we should worry about the surviving party. The Slaves of Chaos will tear one another to pieces, and the Imperium rots already from within.