Some quotes from Forsaken NPC vendors:

"Beware... our enemies abound."
"Beware the living."
"What would you ask of death?"

Some quotes from Sylvanas:

"Who are we but slaves to this torment?"
"We are the Forsaken. We will slaughter anyone who stands in our way."


Recall that after Sylvanas broke free from ol' Lichy, she organized the other free Undead and fought back. She pushed back his entire army, and cleaned all the Scourge out of Lordaeron, something the combined might of all the Alliance was unable to do for quite some time. She was a Ranger-General of Silvermoon in life, the supreme commander of some of the most powerful warriors at the time. She also served as a General for Lichy, purged Lordaeron of all the Humans living in it, created the Dark Rangers, and generally took a whole lot of names and kicked a whole lot of posterior.

After freeing the Forsaken and clobbering the Scourge, the Forsaken wanted to reunite with the Alliance, after all, they were the fallen relatives and ancestors of Lordaeron Humans... but the Alliance didn't exactly like the whole idea of Undead, and tried to kill them. So, the Undead had no place with the Lich King, and their own kin turned their backs on them, so they seek revenge against... everyone.

Some of the Forsaken quests are pretty ruthless. As a Warlock, the quest to gain your Succubus has you track down the kindest Human you can find, a noble fellow that cares for the refugees living at the walls of Gilneas, slaughter him in cold blood, tear out his heart, and use his pure heart to lure a Succubus demon to this plane, so you can enslave it. There is another quest that has you collect samples of water from Azshara, which Forsaken scientists use to whip up a new super-Plague, which you use on a captured Tauren (one of your allies), and watch it die horribly, much to the joy of the Forsaken alchemists. A series of early quests in the Tarren Mill/Southshore area has you slaughter Hillsbrad Farmers, Peasants and collect 40 of their skulls before the Forsaken in town will even give you the time of day. Another quest has you sneak into Hillsbrad, feed a farmer's dog a special concoction, changing it into a massive wolf that goes on to eat the farmer, much to the delight of the Forsaken apothecary who gave you the quest.

There is no doubt about it, the Forsaken are not anyone's friend; the best you can do with them is 'not an enemy at the moment'.