The Forsaken aren't crazy, pretty far from it, actually. They're able to be coldly logical in a way that the living races can't be because death--and, I think, the horror of the way they died--freed them from many of the feelings and illusions that keep us from doing things, like, y'know, love and morality. ;)

***SPOILERS BELOW***

I think Sylvanas' actions in Gilneas and Silverpine demonstrate this best. Her use of the plague on the Gilneans wasn't the work of a madwoman. She was fighting a war, and the plague was the most efficient (and possibly only, at that point) way to beat the Gilneans. That it was wrong didn't enter into it for her. To her, Garrosh was letting petty morality get in the way of victory. A morality that has no place in all-out war, which is supposedly what Garrosh wanted in the first place. Same with her use of the Val'kyr. Her first priority as leader of the Forsaken is to ensure their continued survival. She was absolutely right that without some way to make more Forsaken they were doomed to eventual extinction. The fact that it meant spreading the abomination of undeath is irrelevant. You do what you have to in order to survive, to make sure your people survive.