I know there were exceptions in both time periods. Basically I'm wondering when Mediterranean polities decided on being less cruel in their warfare, being able to adapt to the people they conquered instead of always destroying them.
I understand what you're asking but believe this might be an argument of semantics at some point. Major civilian killing during the medieval era stopped after the 30 years war with the ending of the brutality of the mercenary armies of Europe. However, of the 80 million killed in WW2, the most recent massive European conflict. 50 to 55 million killed were civilians, about half from famine and disease. While this period was well after the Roman period, we see mass killings throughout all ages.
Could you provide some information about where you learnt about this? "Enslaving half and killing half of the people they conquered" was never something that was systematically applied by any "Mediterranean armies", and certainly not the Romans.
Would it be safe to rephrase the question as "when did Mediterranean armies tone down the Roman style cruelty towards noncombatants?"
What is your source for your statement that the Romans enslaved half and killed half of the peoples they conquered? This is most certainly not true.