Originally Posted by
Solonar
They need to rend flesh, spill blood - or to say it short, have to kill to prevent this burning hunger, of which Instructor Razuvious tells the player Death Knight. As Knights of the Scourge, they enjoyed this very much so - but since they are no longer Scourge, they sort of came to grips with who they really are and what's good and wrong. They instead turn this hunger upon their enemies, but it is by no means something they enjoy. (though this can vary from Death Knight to Death Knight)
The text of Razzuvious:
The endless hunger will soon take hold of you, death knight. When it does, you will feel pain immeasurable. There is only one remedy for the suffering: the hunger must be sated.
I give you the key to your salvation.
Chained to the Heart of Acherus are those deemed unworthy of the dark brotherhood. Use the key to free an unworthy initiate. Allow them to equip their gear and battle you for their freedom. Kill and the pain will cease. Fail and suffer for eternity.
Live or die - the choice is yours to make.
This is a bit more in-depth. The Third Gens are created so that they constantly feel nothing but pain and suffer greatly with the only way to stop the pain being to kill. This does several things. The first is that it robs the Death Knight of his emotions, turning him into nothing more than a husk of who he was. The second is that it fills that husk with the urge to kill and cause suffering.
It also appears that this curse can be cured, as the 'agony' forced upon the Third Generation is caused by malevolent spirits only seen in the Shadow Realm. This is evidenced by Koltira Deathweaver's quest, The Power to Destroy - meaning that Death Knights may be able to have a form of positive emotions. Though how good isn't exactly specified - and by the time it is cured..it may already be too late.
(For those who may not know, Third Generation Death Knights are the ones we can play. Second Generation are those such as Arthas, and the Four Horsemen, and the First Generation Death Knights are the orcish Warlocks/Necromancers of old, whose souls were placed in the fallen Stormwind soldier's bodies - these are mostly presumed dead.)
Hope this helped a bit.