Disproportionate response only works temporarily, only when applied arbitrarily (to make your actions unpredictable), and only when applied against a small response. Make it systemic and you have to basically kill absolutely everyone, or you will guarantee a permanent state of war levied against you.

Disproportionate force is simply a force multiplier. It's a weapon. It doesn't win you the war long term. Your other example (executing every fighting age male), is a genocide that assumes you've already won.

My point about Mercy is that it is our weapon after we see an individual has repented. From a tactical standpoint, mercy is about taking someone out of the fight as permanently as if they had been killed, while also being able to use them as a propaganda weapon to guarantee further surrenders.