This is the Hellscream logic :
-My people starves in the desert, i need resources and a new land;
-My enemies have these lands and resources, so i need to kick them from there;
-But while i have to kick them, is clear that they will reorganize and counter-attack, trying to undermine my work and kill my people in attempt of get the lands back;
-So i need not just to kick them out, but crush them, break their spirit, their will, so that they don't even dare to try anything. In this way they will give up and my people will not have to fear any reprisal.
He wants to "destroy" the Alliance until they will not simply give up and submit, because an Alliance submitted and crushed is no harm to no one.
The problem is that the Alliance don't give up, they don't let their spirits to be broken by violent events like Theramore; and if they are persistent in their defiance, then he have to wipe them out from the planet.
Imagination? HAH. Clearly you just played the Scenario without doing the Dominance Offensive quests.
http://www.wowpedia.org/Quest:Regeneration_Takes_Time
Not even our dear Zul'jin couldn't have been able to do anything against it without someone saving his Amani ass.
Rexxar is cool but, seriously? He doesn't have that much influence in the Horde, he's just an "occasional badass champion", not someone involved directly in Horde politics. I bet we'll see Saurfang soon but, still, he haven't had any kind of relationship with Garrosh after Northrend, while Vol'jin has been forced to endure all the Garrosh's reign.