Well that's another beef. Why does the Emperor create a person like Cruze or Lorgar? Angron might have been ok had he not landed where he did. But Lorgar and Cruze were basically preprogrammed to be fuck ups.
And the only answer I have is: plot. These characters were created a long time ago when we knew the Emperor a lot less than we do today. (i.e., before he ever had a speaking part.) As GWS has built up the Emperor over the years, these little bits of cool but honestly nonsensical lore never adapted. It gets handwaved with "he has a plan" like he knew all along the universe was going to end up in the shithole of the 41st Millennium and it was the best he could manage. (Lord knows plenty of other characters in the setting also knew exactly what the future was going to look like, 10,000 years before it happened.) Truth is, GWS wrote some awesome wicked bad guys then had to work backwards on detailing why they actually exist.
It's why I have increasingly mixed emotions about the entire HH. It's trying to justify stuff that cannot honestly be justified when stacked up against how they've built up the Emperor. I mean, dude fucking interacts with the adult Primarchs while they're traveling back through the warp and time to see themselves in their incubation chambers. And this is the guy that thought the Edict of Nikea was feasible?
Horus turning, I can buy. Angron getting dealt a shitty hand in life, I can believe. Perturabo being genetically inclined toward being bitter, I can accept.
But Lorgar embodying the opposite of what the Great Crusade was about though, and Cruze basically being a psychopath because it's in his genes, and the Edict of Nikea, I have a problem with those. Because unless you're drowning in an ocean of Imperial Koolaid, it just doesn't make any fucking sense.