He's paranoid enough to have everything covered, can see the damn future, and is capable of commiting atrocities that even other Astartes and his fellow Primarchs found distasteful. Considering the nature of 40K, and with perfect hindsight knowing what would have been created, I'd have put Curze in charge to put 10,000+ years of fear into the minds of those subjected to his forces violence, and prepared the human race as a whole for the harsh times of the future.
Despite his strong will, it is brittle, and once broken, he could be used; as Emperor, it wouldn't be hard to shore it up and to coach him how best to protect against others, but also to leave that plot hole size chink in his defences for when/if he needed blowing up. The military leader IMHO, should not be the one who is seen as kind and considerate, but one who should be feared by those he faces as well as his subjects. The enforced interaction with others could have cured his neuroses, he could have had a more positive influence on his legion (or the legion's selections at least), and when the silk glove needed to come out, he could pass it on to his other brother Primarchs who were even more greatly loved; Vulkan, Horus, or Sanguinius, say.
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