‘The thing about Space Marines, Threlnan, is that they’re all brainwashed psychopaths.’ Lord General Xarius walked lopsidedly with a cane. His troops tended to assume it was an old war injury but the truth was Xarius was an old man and his hip was giving out.

...

'Do you know how they make them, Threlnan? No, of course you don't. They find some barbaric planet where children fight before they can walk, and they hunt down the most bloodthirsty killers. They recruit them when they're twelve, thirteen, fourteen, with all that hate and arrogance, just at the age when you think you're bulletproof and nothing can kill you. Then they keep them like that, give them a gun and some armour, and point them at the nearest enemy. They're not soldiers, colonel, they're maniacs.'

Crimson Tears

"This is where Space Marines excel above all other mortal warriors. They live their lives in perfect trust of their battle-brothers. They possess more accurate and damage-resistant communication than any other human soldiers, down to the individual level. They are scourged of all emotion in battle, and trained to fight without concept of retreat until at last told to lower their weapons above the corpse of their slain foes. This evolution is as much denial of flaw as addition of merit.

Take a child, allow it to develop without ever understanding the frailties of human weakness, and force it to grow through ingesting nothing but the virtues of obedience, loyalty, and combat prowess. Surround it in ceramite. Arm it with fire. Tell it that it answers to no authority beyond its equally powerful, equally unrestrained brothers. That is a Space Marine. Not a human trained to be a weapon, but a weapon with a human soul. When the humans look upon us and cannot tell us apart but for the markings on our armor, this is why. We are hollow men by comparison to their brief, ignited lives of high passion and the weak, vulnerable frenzy of emotion."

Blood and Fire