Board Thread:Warhammer 40k General Discussion/@comment-5.65.81.205-20131020155613/@comment-71.218.152.127-20140511063412

Since daemons are merely agents of a Chaos gods' mind and will, it is not possible to kill a daemon unless the chaos gods' mind is damaged in some way. Daemons could of course be kept out of realspace by a psychic shield and would be trapped in the warp, making them essentially dead. It is also possible for a daemon to die in the warp and not live on unless a chaos god reanimates it.

And I also have another theory which I can't really prove but it's simply a theory that there was no recorded daemonic activity before the Emperor rose to power. It could be possible that the chaos gods needed worshipers to be able to fuel daemons, it should be noted that the chaos gods did know of the Emperor's presence on Terra before the Unification wars, why Daemons did not attack the Emperor or his servants before the Great Crusade is unknown in that case. Like I said that's just a theory I thought about so don't hammer me for it.