Not so much human nature, which implies foibles, but nature in general. And to blame human nature for the failures of Communism really is just like wishing the sun would revolve around the Earth just to be in the right.Castro and all the others will realize that though equality has a very nice ring to it, human nature will never allow such a system to come to be, it is impossible and always doomed to failure.
A basic example of the inherent inequalities in nature is the fact that some men are stronger whilst others can run faster. Training helps but nature defines.
In terms of brains, some men are capable of using their loaves to become super-successful businessmen whilst others only have the acumen just to do an ordinary day's work.
Not being a communist allows greater freedom of initiative and talent in whatever area you're good at.
Communism can only mean denuding people of free will and the right not to be managed like robots off a production line.
That's why Castro and all the others are absolutely wrong. Total equality can only ever be imposed if not undertaken by a few people who happen to all be the same anyway.