Here's an interesting one: Kel'Thuzad. While he is definitely not a "good" guy, his motivation is highly relatable and justifiable.
>Be a powerful and magically gifted individual who can come to a position where all the rot of the human kingdoms become apparent: labourers are overburdened and exploited while imprisoned Orcs are held in concentration camps. All the while human lords grow fat and abuse their power.
>Attempt to expand the known magical knowledge towards necromancy but get screwed over by the dogmatic order of the mages.
>Hear a powerful figure reach out for you. Investigate, discover the true power of the Lich King, and realize you need not be a servant, but can use this as a means to an end.
>Swear fealty to the Lich King and start spreading your own revolutionary doctrine throughout human lands: let us do away with the lords, with the social inequality, with death. Let us forever more be immortal and equal, in service of our new king, throw away the dogmatic superstitions of the Light and embrace undeath!
>Recruit many like minded followers. Eventually set of the events that will scour the corrupt old order from the face of the earth.
>To keep you is no benefit, to destroy you is no loss: all those who didn't accept the gift shall be wiped out by the plague and newly raised armies of undead, forced to serve the will of the master.
>Get killed, only to be resurrected and ascend to become a lich latter due to you invaluable service.
>Endear yourself to the princeling Arthas, make him your friend, and a suitable replacement for the original Lich King.
>Free the Scourge from any demonic control. Now it dictates its own future.
And here is his manifesto, "One Truth in Undeath":
<Behold the finest of ironies. The living ascribe unto undeath all the most sickly attributes of their own lives--hopeless servitude, savagery, stillness of the spirit. We serve! The living serve, too, though they serve kings, warlords, druids, priests, gods, men, and beasts. We serve only the mighty, unifying will of the Lich King, which compels us unto prosperity and unity!
<For all the endeavours of men, there exist only endless records of tragedy, cruelty, betrayal, and selfishness. For the Scourge, there is only efficiency and totality. Serve the Lich King in life, or serve him in death. His is a way of choice whose roads lead only to fulfillment!
<Cold in flesh, but warmed by unity, the mighty Scourge are one nation, one mind, one being. True cold dwells in the heartless gaze of the guards who turn away sickly refugees from their neighboring country in a time of war! Are not all men of one kind? Nay, divided in their icy hearts.
<Raise high our ideals, and bring low our foes! Fold their broken into our number until all serve the Frozen Throne!