Ares created a shitty armistice that would lead to an even worse war. Don't sleep during films Gaffers.
It's all just kinda lame misdirection and a convoluted scheme for a twist that doesn't have much payoff ("The minor character you paid no attention to is actually the bad guy"). I agree with the critique that "secret villains" is becoming kind of a tired trope. Introducing the villain in the last 15 minutes is a lot of heavy lifting for a film to do, though it works better here because we're honestly not sure if Ares is going to turn up at all.