I'm starting to think that, as quite a few posters have pointed out, it may have been something as simple as someone telling him "no".
"No, you can't spend $300k on the family money on a painting of a black penis".
Someone genuinely putting their foot down and denying something to him shatered the illusion that he was daddy's special little boy, at the height of his e-fame at that, his planet sized ego deflated into a shriveled prune of a man and so did his physique follow.
All you have to do is photoshop his tongue-out face onto Lucius the eternal, they must have been separated at birth or something.
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Add a variety of rekieta.png on his chestplate for good measure and you have your Lawchampion of slaneesh.