Death comes to you. Feel it's icy breath!
You have no purpose but to die by my hand.
RRAAAAAAARRGH!
Fall, creature, and feed the earth!
Now I have become death, the destroyer of worlds!
You will suffer! You will all suffer!
Ooh, Baldur's Gate quotes! (Yes, I know
Now I have become death predates BG).
How about:
"I will be the last! And you will go first... Heh hehehehe..." (more Sarevok)
"Heh, I do not fear death... do you?"
"I wanted infravision like the elves... But 'tis more than just taking their eyes..." (more Xzar)
"Those the Gods wish to destroy, they first make mad, mad, mad!"
"So the stoic adventurers have found their way down to my lair." (Davaeorn)
"Please don't disturb me while I'm plotting to overthrow you." (Edwin)
"Me will crush you! Crush you to GOO!" (Ogre)
"You mess with me, you mess with YOU!" (Ogrillion)
"I am death, come for thee. Surrender, and thy passage shall be... quicker." (Nimbul)
"You dare to attack me here? Do you even KNOW whom you face? You will suffer! You will ALL suffer!" (more Irenicus)
"Must I be interrupted at every turn?! Enough of this!"
"You are but a gnat, compared to my power."
And just for fun:
"Oh, what is the point?" (Xan)
"Life is so hollow."
"Let us save our effort and just lie down and die!"