I disagree, the warp exists and is fueled by the existence of mortal life but the Chaos Gods require worship to sustain themselves you need to kill in the name of khorne or one of his countless demons with the belief that your killing is holy worship to the god of blood otherwise the gods wouldnt've been starved by the great crusade and wouldnt've joined forces to stop them, the crusade should've empowered the gods to pornographic levels but it did the opposite..
Killing for the sake of killing and/or to worship a demon fuels Khorne but killing for another purpose with no warp entity in mind wont give anything to khorne.
i disagree, there is no evidence that the Great crusade in anyway significantly weakened the gods, the crusade was but a means to an end , the end being the webway and the advent of an enlightened humanity, with the guidance of the Emperor and throughout countless years, the Emperor would curb Humanities wanton lust for war, its indulgence in excess etc... thereby weakening the gods, the great crusade was just to unify everyone under his Aegis.
Think of the warp as paint, pure white, that is its natural form, then with creation of psychic races the paint gained color, each color represented an emotion the weaker emotions had correspondingly weaker colors (lasting for a short time), the stronger emotions have a stronger color (lasting for a longer time), if a berserk loyalist space marine killed heretics and spilt their blood in a ravenous blood crazed manner, regardless of his intentions it would paint the warp red, since its a powerful emotion its a dark crimson, in a similar way a bloodletter of khorne decapitates his enemies for the the pleasure of the blood god painting the warp red, different intentions yet both still fuel the gods, they are emotion given form, they are excess (because stronger emotions last longer and are more powerful).
before the birth of khorne, slaanesh and the others, no one actively worshipped them, yet they came into being, no one said "oh yeah this is for the blood god" they just killed and indulged in excess and carnal pleasures, as Rems said they coalesce from raw emotion not worship.
if the above sounds retarded im sorry, i have a fever.