Once, goblins were kind beings, living only to further the good of the world. They were outmatched by the immortal elves, the advanced dwarves, and the numerous humans. Only the unintelligent kobolds were weak enough to be receptive to the goblins, and they were too scattered and ununited to help in any way.
Then, some being came to them, and offered them gifts. They would live in constant pain, but would never die and would be able to create obsidian towers and iron tools. This was judged as an acceptable sacrifice to the goblins, however, who mostly grew used to the pain.
The ones who made the pact, that is.
The next generation of goblins was twisted more by the Pact, in mind and body. Instead of fair, pale skin, these Pact-Twisted infants were marked by green flesh and worse deformities. These mutants, called Abominations, misinterpreted "The Greater Good" as "Good for Goblins" and resented their elders for for the curse brought on them, and lacked any true morals. The old generation was killed, and the new generation took up the mantle. They began to loot and burn the surrounding lands, and with their dark powers and iron weapons, they expanded their power. The Abominations did not expect their downfall: Their parents.
The slain generation had been left out on the ground to rot and be eaten by birds, and their spirits were not put to rest. These spirits finally chose to possess the bodies of their unborn grandchildren. This generation resembled a blend of the Ancient Goblins and the Abominations, and were more or less identical to modern goblins. While they kept some of their parents' ways of looting and kidnapping, these New Goblins had--and have--the altruistic mindset that governed the Ancient Goblins. While they don't have the power of the Abominations, the New Goblins do not age. The Abominations mostly died, in battle at the hands of the New Goblins, but others fled deeper than any race had ever gone. They bred with the demons of that foul place, and some of these humanoid half-demons escaped back to the surface to govern their New Goblin cousins or even humans, who were the least affected by the wars the Abominations waged. Now, as dwarves dig deeper and deeper, they risk finding the dark secrets of the goblins' immortality and the death that they will bring to them...
That goes nicely along with my Tau-like image of the goblins. Think about it--aside from elves, no other race is allied with any "lesser" sentient race. The Greater Good? It fits goblins like a +<<*troll fur glove*>>+.