The Forsaken have two defining features and one goal.

1) Free will. Blizzard, regardless of tinfoil hat theories, are not very subtle about this to the point where in game text has actually been changed to make this clear when it wasn't apparent from people shouting at you. Every single Forsaken is a creature who managed to extract themselves from the Scourge hivemind and joined the Forsaken of their own free will. Every single forsaken is given a choice to hang round or not. And hey, Will of the Forsaken and highest spirit out of any race. These are people who, for the most part, aren't doing anything they don't want to. Note free will does not apply to non-forsaken on account of attribute 2.

2)Ruthlessness. The Forsaken aren't so much evil (Though they are pretty evil) as pragmatic. If doing what they need to do requires a huge pile of corpses, then a huge pile of corpses there shall be. If it requires building a deadly poison gas, then poison gas there shall be. If it requires a huge pile of cockney corpses produced by poison gas, then no one will care. Because a cockney, werewolf or otherwise, isn't a real person. But 1 prevents two from turning into Arthas or Kael or what have you because they aren't going to suborn themselves to a greater evil. They're just going to be dicks all on their lonesome.

Both of these work together to make 3) The Primary Goal of the Forsaken is the survival of a forsaken as a people and thanks to 1 and 2, you should probably not get in the way of this. You can put whatever reading you want into Sylvanas's little speech in Silverpine, but given it's the forsaken starting zone and it's meant to orientate you to your faction themes (Free Will. Nobody Likes The Forsaken. We Tend To Be Dicks, We Would Like to Continue Existing, We're Not Taking Anyones Crap and Hey, Forsaken Are People Too, Just Dead People. Also, Belmont Is Surrounded By Idiots), and given that blizzard to operate at the level of subtlty of a mack truck in everyone elses case, it means what it says on the box. Everything the forsaken does springs from these three factors and in that respect they're no more evil than, say, every other race in the game. With just the unfortunate side effect of being zombies.