The Emperor bio-engineered the Space Marines to be the perfect soldiers. They are immune to fear, untroubled with weaker emotions, and unflinching the face of destruction. Taken as children at ages ten to fourteen, each Space Marine underwent a grueling trial and process to become the living man weapon that he is today.
What if anything was lost in the process? Space Marines are no longer strictly speaking human. They were granted powers and abilities far beyond that of mere mortals. They do not seem to die by natural processes, are spared the indignity and feebleness of old age, and are immune to normal disease.
Along with the undeniable gifts, the process also expunges many irreplaceable, essential human qualities. A Space Marine loyal to the Imperium cannot love and cannot feel romantic desire. He is effectively castrated, and cannot have children. He is destined to be a killer and a destroyer. He is not allowed to remain peacefully on his own planet, but is required to raze and conquer other worlds, whose sole sin may be to wish to be left alone and govern themselves. He must further commit genocide in Terra's name, because his master deems Xenos too different and imperfect to co-exist with in the galaxy.
He is effectively rendered into a human lab rat, so Apothecaries can mutate his body to sprout additional organs and glands in accordance with a barely understood or controlled science. In the case of Space Marines with tainted gene seed, they may also be subject to the dark spectre of genetic defects such as the curse of the wulfen or the red thirst. These choices were effectively taken away from a Space Marine before he was of the age to properly appreciate or consent to the change. Every Space Marine is essentially a child soldier, preordained through genetic manipulation to be a murderer.
The Emperor's Children are unusual post heresy insofar as they have chosen to no longer follow the blueprint laid upon them by the false Emperor. They choose their own desires and plot out their own destinies. No longer confined to the restraining orbit of the Emperor's pre-programmed pathways for his own benefit, the Emperor's Children are free to seek their own evolution and improvement as they may independently see fit . . . . whether that is in the form of ultimate experiential hedonism, artistic complexity, martial perfection or otherwise.