Give me one reason why we shouldn't just kill all men?

Let's see your average simp square up to yoir average Chad, lmao
I'm only 5'10"/165 lbs, and I could knock seven shades of shit out of an average zoomer simp - so I don't even want to imagine how badly your simp army would get beat by men bigger than I am.
And in return, instead of "sparing us", you would spread your legs wide open for the minority high testosterone men who would be left over - in fact, because of the newly lowered ratio of men to women, you'd have to go out of your way to earn the trust of a real man, instead of stumbling through life with the assistance and attention of beta males.

They asked why we shouldn't not why we couldn't. And all you did was give just another reason for why we should (men being morally inferior and more likely to commit violent crimes)
OP is just a larping moid though.