Well, you see, if men stay alive, but women are killed, society collapses eventually as not enough new people are born. It sucks being a man in this scenario, but it is what it is.
If you all agree to refuse to fight, you win.
https://en.wikipedia.org/wiki/Prisoner%27s_dilemma
The key here is to refuse fighting. Nobody becomes a hero by becoming a murderer whose goal is to defend the political power of Stalin, Napoleon, Bush, or whoever.