While interesting I think the problem with the defect theory is that Sanguinius was also a mutant with his wings and he is supposedly the most beloved of the Primarchs. Still it's an interesting theory. Though I do find one of WH40k's main problems is the story goes nowhere ever. Fluff gets updated or rewritten but the actual story is basically frozen with no major events ever happening. While I'm not saying I could do the job if I was a writer there I would probably be trying to move the story forward with the Primarchs or through having people start actually trying to think. Still my problem is I haven't read all of the fluff, codexs or books so I don't know all of the events that have happened. I have an idea for a chapter though I should get around to writing that whole story down at some point.