You may have already noticed that the orcs are very short-sighted. They only care about conquering and pillaging, and still they seem oblivious to why the imperials hate them. They are pretty much like Attila the Hun, who loved to cause chaos and death, but that is just it. Unlike the humans, they don't think "outside of the box".
After the undead kill most of their chieftains, rather than regroup, they decide to attack the imperial fortress anyway and after the demon attack, they are forced to retreat, making their whole plan an absolute failure. The orcs ended up more weakened than stronger.
Vanessa was right, coexistence or at least not killing all humans, could guarantee the orcs' survival and victory.