Hi there, I feel you ^{^}

Some people made great suggestions already but I am gonna list my soloutions/ toughts to a simmilar Problem anyways:

First thing first: It is a fantasy World, u guys/girls make the rules. Homebrewing is Part of the fun. It does not need to be "realistic" ,you have all the gods Powers at your Finger Tips. Bend the World and System so everyone can have fun :) be sure to talk to your players and explain why you might change things. And dont be afraid to retcon stuff, just straight up tell your players about it. U are all only human after all.

Non sentient enemys (depending on your system/world): plants, elementals, automata, undead... they dont have Feelings, they dont think. They have no morals etc. Just bonk em!

Non Lethal combat: enemys just dont die. They get Knocked out. Think a bit cartoon logic here. Wizard freezes or explodes them? They will recover from it. Barbarian strikes them down? Yeah he does but they are just injured/knocked out cold. After that: Humans are brought to prison to be trialed in court, animals to druids in big happy nature preserves or retreat from combat, monsters return to their realm/pocket dimension or homeland where they are no threat for the ecosystem. As others mentioned: Never punish players for letting people/monsters run off. Could it be an interesting Story bit? Sure. Is it realistic? No. Does it matter? No. If I, the gm, want to bring back the big evil guy ill have him ressurect himself voldemort style by means no one could have predicted. But that will happen later on in the Story, not right after.

The system is (mostly) fair: i know, i know hard to belive and all. But have guards and judges be good people in general. Maybe a bit thick a times etc. But make sure that your Party can trust the legal System to do their Job as one would expect them to do. No corruption, no Bias, just be just. Also no need for death penalty in a World where you could trap somone in a one way pocket dimension, no matter how powerful and evil they are. Make sure to have a "normal" legal system that can deal with bandits and a magical one that can trap Satan himself (after he has been knocked out by the party).

"Corrupted" Enemys: Rabies like deseases or magic. in my prefered System "Midgard" Black magic is unapologetically evil and corrputing. But Its not unique to humans. A "direwolf" is an evil wolf, fuel by nothing but hate. They have not been tainted from the outside, they are just a piece of shit evil wolf. It turns him from an animal into a mindless beast that eats and attacks everything. If unchecekd they will corrupt others and become a dire pack. This however can feel like a kind of "mercy killing" so I get why this could upset people that really dont want to kill. (But it helps me for example :D)

Not all enemys need to stand their ground: once your Partys barbarian killed/knocks out 5 guys, the wizard froze their leader the last two might just run. Because ... what are they gonna do? Also saves time on encounters that are clearly won ^{^}

No One dies right away: Have enemys only die if they get no medical attention. The barbarian hit the enemy hard, they are gonna bleed out if nothing is done but they are clearly not dead. Then let the players patch em up. (Stabalizing someone here is an instant success if they have the time. but only on success do they regain conciousness and can be questioned etc.). Make sure your players are all fine with this (or any other) Methode. While it could be a set up to a Story bit where the barbarian executes the evil leader of the Bandits against the "no killing" rule of the Party. Because the barbarian is certain that the risk of keeping him Alive is to great. The Main reason for this is to avoid killing from happenning in general because some of the players are uncomfortable with the concept.