“Hola skinks!” -skinks
“A good flay to die!” -Ikit claw
“WAAAAAAAAAGGGGGGGHHHH!!!!!”-Orks
”It’ll take a lot more than that to kill a-Auugh!” -an Ork, after being hit with a sticky bomb
“Alright lads, Gorkamorka wants us to kill them shiny ‘humies over there! Let’s get ‘em!” - Ironjawz
“Oh, there WILL be resistance to our ascension, of course. But if the blood of unbelievers must flow... then these streets WIILL. RUN. RED! We are a horde; a tide; an army born in the shadows! Humanity has always looked to the stars for enlightenment, and at last... THE STARS HAVE ANSWERED!” -Genestealer cults
“And what are the achievements of your fragile Imperium? It is a corpse rotting slowly from within while maggots writhe in its belly. It was built with the toil of heroes and giants, and now it is inhabited by frightened weaklings to whom the glories of those times are half-forgotten legends. I have forgotten nothing and my wisdom has expanded far beyond mere mortal frailties.”
“Please forgive me the horror we both know you will make me put you through.” -Ahzek Ahriman
“What is Chaos? Suffering, you might say. Oppression. Deceit. But could not all these things be said of your Imperium? You hunt down the talented and the strong-willed. You break them or sacrifice them. You lie to your citizens and wage war on those who dare speak out. The inquisitors you call masters assume guilt and execute millions on a whim. And why? Why do you do this? Because you know Chaos is there but you do not know how to fight it, so you crush your own citizens for fear that they might aid the Enemy. The Imperium suffers because of Chaos. No matter how hard you fight, that will never change. Chaos exists in a state of permanent victory over you - you dance to our tune, mortal one, you butcher and torture and repress one another because the gods of the warp require you to. The Imperium is founded on Chaos. My lord Tzeentch won your war a long, long time ago.” -Ghargatuloth, daemon prince of tzeentch
There was also a great quote from the core book about the stormcasts. It went something like “As they started to become more and less than human, a question hung in the air, but none dared ask it aloud: is it possible for a man to be so pure, so holy, that in the eyes of a mortal, he would be the embodiment of terror?”
I wrote this on my phone, so sorry if it looks weird.