At the moment, it seems that the vast majority of people carve out a special ethical exception for the military but I suspect this will change over time. Many futurist/sci-fi dystopian scenarios have the general shape of the Terminator movies with autonomous killing machines roaming the planet. If these proliferate, perhaps the terror of that reality will bring into focus the immorality of using force to achieve geopolitical goals. And it feels like we're getting uncomfortably close to that future.
Good point, the military is a necessary evil right now, but it could be disbanded if we learn to solve conflicts the proper way. I see robot vs robot battles as and intermediate step in this direction.
"The purpose of the military is to kill people and break things."
- Gov. Mike Huckabee and former Republican Candidate for US President
I don't think robot soccer is going to be the intermediate step since the threat of human death and the follow through on that threat is an essential feature of the war machine.
As difficult to get belligerents to agree to ethical rules of engagement, which would be a requirement for war as robot sport, we can't ignore the rise of asymmetric warfare and warfare via proxy through militias and unofficially backed terrorist organizations. These are often used by states as loopholes to get around treaties and international law. Are terrorists and militias going to be adhering to something resembling Asimov's laws of robotics when deploying their autonomous weapons?