And that's where the tricky part begins. Actually, I hate this word, it's childish. Everything has a motive and consequences, but simple cause-effect relations get twisted and uncontrollable at times. That is why they say that the road to Hell is paved with good intentions.
But regarding the Forsaken, one should remember that they have a completely different view on death. Slaying someone isn't a problem because they can be simply rezzed to see the humor of their situation. You may loathe undead, but let's look at how you sing when you become undead. This may seen a small thing at first, but I think it's one of the fundamental things in terms of morale.
As for the plague, I see no problem at all. Yes, it's effective. It's a weapon of mass destruction that does not damage non-organic substance, and it's awesome. In fact, so awesome that Orgrimmar is afraid of Forsaken. You may say anything, but this kind of weaponry is way better than what is used commonly - cannon fodder that mindlessly shouts "Lok'tar Ogar/For the Alliance!" or explosives that destroy absolutely everything. Personally, I don't care what brings death - be it a blade (which is somehow considered valorous), bullet (considered normal), explosion (considered a bit gruesome), or disease/poison (which is somehow considered dirty and immoral to deliver). Death is death, doesn't matter how it happened. Dumb knights think it's low to use a disease to kill one of them because "it takes no skill", go brew it yourself then.
It's not called "betrayal", it's called "foresight". Do you honestly think they didn't think it over? It was crystal clear that Garithos wanted to simply exploit the Forsaken and get rid of them, leaving them nothing in return, you don't have to be a genius to understand it. Sylvanas simply chose to use him instead, preventing any bullshit he would without doubt rain down on them. Sould I remind you that it started with him "thanking" undead Lordaeronians for freeing him of slavery by allowing them to help him claim the Capital and then get out with no reward. How's that called?
Would you always wait to be abused just to be able to tell you weren't the first one to strike, and therefore innocent? This principle can also be played with easily, just set youself up to look like a victim. Sylvanas simply didn't bother with that.