[@GreivousKhan]
Runes act like the “battery” for the magic. They only start absorbing mana when they are painted with blood, death or sense nearby fear and all that good stuff. If you want the adverse side-effects to only apply to the usage of runes, then I would propose this concept:

[i]Grave Sickness[/i]
Death can be a consuming force, even for the living. The frozen grasp, the maddening howls and the decaying rot do not merely destroy an opponent. They affect the user as well. Making them sickly. Early stages are marked by pale skin and haunted dreams. Later on the eyes recede into the skull. This is mostly when the detachment happens. Dread Knights all suffer from a level of disillusioning towards the world. Suddenly nothing in their lives matter. A great many wives have divorced their husbands due to this inability to care. But the sickness doesn’t stop there. Death Knights, those veterans that have been using the runes for many years, grow ever more distant from civilization. Priests of Death will rarely be seen within cities, often opting to sleep in graveyard crypts or near monuments. When not in combat, a great many Priests look sickly and weak (they are, physically not weaker than before. However, they are far more haunted by passed spirits and memories of corpses). Some lose all sense in their toes and/or fingers after a time of feeling bitter cold (even though they try to warm themselves before fires). They have little concern for the healthy living. If they care for someone, it is when they are dead. Or on its doorstep. In practice this would mean that a Priest would sit still while a village is being raided, uncaring for the many inhabitant. Only after the raid would he go around to mercifully kill those wounded that are too far gone. However, should a raider make the mistake of attacking the priest, the priest will assume the raider forfeits his life and will slay him in an instant. Those so far gone are often marked with sleek, black lines running over their body. While they are generally uncaring towards anything that lives, they do honor their oath to Alexander. Though only him.