You seem very biased towards the Alliance side of things.
You are portraying that the orcs just wanted to kill everyone and take the land. But you are failing to mention the motives. The orcs literally had no choice the first time because it was either "we kill these humans and save our race" or "we stay in our dieing world and slowly be killed".
With Garrosh, it was purely the work of a dictator. He literally killed the Tauren leader because he went against him. So can you actually blame people for not wanting to get on his bad side?
The orcs have been controlled and manipulated by someone a lot. Why is it unreasonable for them to want peace when they are currently not?