Really, what good reason could exist for the Royal Apothecary society to be developing a plague that would exerminate undead and living equally efficiently if they didn't intend to...use it to kill people?
Well, right. And killing is wrong, certainly, but they're using it in war. Putricide had help from Sylvanas, but as I recall, it was under the guise of using it to kill humans - their enemies.
We have to remember, when looking at Warcraft, that we the players are asked to do terrible, horrible things for gold. I mean, we were asked to wipe out the Amani for 'the shinies', and some quests have us decapitate people (thieves and mercenaries, for instance) just for disagreeing with the quest giver. The players are, arguably, evil, just by virtue of doing most of the quests. As a note of interest, here
is a link of all the 'head' quests you can get.
I agree that the forsaken are, in large part, sociopathic and evil, but so are most of the quests. I mean, anyone who did the quests in Venomspite helped the forsaken to hatch their plot, no matter what race they were. I understand that players /= lore, but still.
I never played the alliance for very long but I know there were quests I couldn't really have my character do because, well, the alliance was asked to do some pretty terrible things as well in quests. I just can't think of any off the top of my head at the moment.
If we're comparing Warcraft to our own standards of morality (such as, in real life, I feel that killing is always wrong, however in Warcraft there are beings of undeniable chaos and evil that seek to eradicate all life which I would be (more) okay with killing) then we're missing a lot of the things that make Warcraft different from our own world, such as beings of undeniable good and evil that seek to corrupt and help, meddling in the affairs of sentient beings.
Would you say 'all humans are evil because humans have used mustard gas in times of war'? I mean, mustard gas was created to kill, but I wouldn't say that the creation or use delegates all humans to 'evil' status. Same with, say, nuclear weapons and napalm and guns, etc. Humans in our world vary on the spectrum, and I don't feel that any sentient race in WoW can really be defined one way or another, unless you're looking at the legion troops/demons.
As for 'no good apples' in the forsaken, I'm not certain on this. There's lots of forsaken who seem to want nothing more than to live a normal life - say, the ones that run the inn in brill, or food and crafting vendors. Sure they don't have a huge and important role, but they're certainly not going out and killing everyone. They were corrupted by unspeakable evil power, but they revolted, and some are bitter and vengeful, but others seem to just be trying to make the best of a bad situation. Sylvanas, as well, helped to kill the Lich King whom was a much bigger threat than herself, and though she did it out of a need for vengeance, she still did something 'good' for the world.