By your logic, humans are bad because they slaughtered everyone in Stratholme under Arthas.
They didn't want to do it (for self preservation). Lydon tried to oppose Garrosh, but he was too weak to do it. It's not like they could refuse when Garrosh has an entire Horde army there.
Also, they have a genuine grievance against Gilneas which Garrosh used in his speech to rile them up. Gilneas abandoned them to die in the Third War.
They are doing it for her. They willingly sacrifice themselves to keep her alive. One sacrifices herself to resurrect Sylvanas at ICC. Three sacrifice themselves to resurrect her in Silverpine. And they aren't going to a peaceful afterlife. They knowingly taking her place in eternal torment.
If it was a fabrication, how exactly would they be aware of what is happening at Gilneas?
Gul'dan didn't order the attack on the Draenei, it was Ner'zhul. They were given this information from Kil'jaeden posing as one of their benevolent spirits (his dead wife). What reason do they have to question their spirits, whom have guided and protected them for at least a thousand years? Once the spirit began showing intense hatred for the Draenei, Ner'zhul began to question the spirit's divinity.
Drinking the Blood was something they did out of a lust for power. Not the best reason, but they didn't know it would rob them of their morality.
The vision shows the Alliance invasion goes through the East side of EK and attacks UC from WPL. That's why she's trying to secure those locations.
Their lack of emotion is a sign of psychopathy. Being a psychopath is not the sole determinant of a person being evil or doing evil things. There are plenty of psychopaths who aren't murderers.
Also, it is possible for them to show concern for other people (even the living). Lydon was concerned about what would happen to the Garithos' men after they captured Lordaeron.
---------- Post added 2013-02-17 at 01:09 PM ----------
Just because they didn't lose their free will doesn't mean they weren't slaves. The Blood Curse corrupted them and robbed them of their morality and inhibition (involuntary intoxication like drugged up sex slaves). This made them pliable to the Legion's will. Once they were corrupted, Kil'jaeden had them reproduce and accelerated their growth into adults, robbing them of their childhood. This massive population boom ravaged the resources of the planet, causing infighting. An entire generation was bred by the Legion.