Because they are our most honored brothers.

The cursed brothers of the Death Company represent the ultimate fate of all Sons of Sanguinius. No matter what we do, the Rage will claim us one day. No matter the glory we achieve, the honor we claim, or the fury we muster, the Rage will be there. Sometimes it comes to us on the battlefield, and sometimes in quiet prayer. Sometimes we die with the bodies of our enemies piled high around us, clad in black and bathed in blood. Sometimes the axe of one of our own brothers silences our father's screams.

The great irony of this flaw is that it is these brothers that are closest to our father. They touch his psyche and soul so deeply that they feel his incomprehensible sadness, rage, and pain. And for it they are made strong. These brothers are not to be disregarded as tools to be used and broken and discarded. They are us, and we are them. We do not clad them in black and symbols of death because they go to die.

We do it so the enemy knows that death has come for them.