In a 40k context, human morality is petty by comparison because it's one of several competing theories - Eldar and Ork morality are no more or less valid, for example - and because it considers only humanity.
Imperial morality, I grant you, considers only humanity. I don't see how it becomes petty simply because of the existence competing moral systems. The same logic leads one to label all morality petty, whether alien or a human system.
On a galactic scale, or when considering the warp, morality
is petty. Morals being a construct of society (and one which changes as the context it exists within changes), they cannot adequately encompass things vaster than the societies from which those morals originate.
To human morality in the 41st Millennium (that is, the morality of the Imperium of Man), genocide is an acceptable response to territorial disputes. Humans are, ostensibly, 'good' primarily because they are human, so long as their behaviours fall within expected boundaries. Non-human sapients are fundamentally deemed 'evil' by that same morality, irrespective of the manner in which they behave.
I'm prepared to accept the idea that the Empyrean is essentially ineffable, but Chaos can be experienced though it's manifestations in the real universe.
Do the acts of a believer define a religion? Certainly, while the servants of Chaos may engage in deeds that may subjectively referred to as evil (where 'Evil' is a notion defined by morals, which are in and of themselves, unique to a given culture and thus subjective), their doing so is a matter of their interpretation of the needs of their patron, rather than necessarily being the irrefutable will of the Ruinous Powers.
Beyond that... the Immaterium is anathema to the structures and fundamental nature of reality - reality is order, driven by rules and mechanisms that continue inexorably, while the Immaterium is formless, shapeless and entirely mutable to the designs of any with the will to exert upon it. Manifestations of Chaos in the material world will be, due to their nature as extensions of the Aethyr, inherently destructive to the world.
The way I see the Ruinous Powers, they are the spiritual manifestation of the sins of Mankind (And other races … though I’m sure the teeming billions of Mankind have a significant influence on their nature).
Technically, it's likely closer to teeming quintillions - on a previous incarnation of the Conclave, Helst and a couple of others worked out an approximate population of the Imperium of Man, which was somewhere in the region of 3.3x10^18 (330,000,000,000,000,000) human beings.
Beyond that... the view's a little humanocentric, don't you think? Afterall, for all their bluster about dominating the universe, there is no guarantee that human beings are the single most populous sapient species in the galaxy. Orks may well have a legitimate claim to that particular title. Also remember that it's not raw numbers, but rather collective psychic magnitude that defines influence upon the Warp.
Chaos certainly has a lot riding on mankind, and appears as a dark reflection of it more often than not... but consider that Chaos has existed since long before mankind ever did, and the only Chaos God created since the rise of humanity as a psychic species... was created by the folly of the Eldar, rather than by the collective ills of mankind.
For me, Chaos is emotion, or rather, the resultant psychosympathetic reaction of the tides of the Immaterium upon reaction to the emotions of sapient creatures. Every stray thought, every dream and nightmare, every idle imagining ripples through the Aethyr, and where particular thoughts and emotions are particularly strong or frequently reinforced by similar feelings, they coalesce into something greater.
Khorne is at the core a representation of and gestalt pseudoconsciousness formed from all anger, hatred, defiant pride, aggression, etc that has, is, and ever will be felt by any mortal creature anywhere in the material galaxy reflected by this particular corner of the Warp. Nurgle is similarly despair, hopelessness, acceptance, tradition, inventiveness, reliability and simple joy. Tzeentch is change for better or worse, manipulation, deceit, hope, imagination, and the desire for progress. Slaanesh is the pursuit of pleasure and the desire for sensation or fulfilment. Everything an individual is can be found, somewhere, within one or more of the Chaos Gods if you look hard enough... the good and the bad. Chaos is destructive because it is emotion without restraint or compromise, Id without Ego or Super-ego. Slaanesh represents it best - when done in moderation, the pursuit of pleasure is beyond objection... but to seek pleasure to the exclusion of all else, with no regard for taboo or cost, is dangerous and destructive.