There is one problem in that, the undead. They aren't weapons of the Horde, they are members of the Forsaken. He considers the undead vile beings, but he has no choice but to accept them.
Tides of War is the defining moment. His grand victory is taken way and he doesn't take it well, at all. That's the moment he decided to use all available resources to clean the Alliance from Azeroth. After he used a Manabomb, Magma Giants and Krakens. It's exactly like Golden said, Garrosh has a weak personality with an enormous need of validation. Validation that keeps being taken from the defeats he suffers.