Solterra is not having an ego moment. Neither is he slaughtering priests, nor starting a war. Cole wants to take Therral land and hold Alexis hostage from inheriting the throne. The armies are ordered to kill the soldiers. They will try their hardest to leave the innocents alone, unlike Cole. But Solterra will end this war, whether it is with him going down in a fight, or taking Cole down. 

Win or lose, Cole's forces will be brought down. Either Solterra gets land, riches, and fame and he saves his sister, or he won't be witness to the possible harm that could come to her, or the fall of Therral. His men are not fighting for a crown, or for riches. They are fighting for their homes and their families. So if Cole really wants to do this, he better be prepared for the consequences, win or lose.