One of the best ways to show your PCs that running away or surrendering are viable things to do in the game is by having NPCs try to do it. When an NPC sees a fight is turning against them and they start to run, the PCs may get the idea that it's something they should do. When the last NPC offers to surrender, the game can take an interesting turn. Now, your PCs may just kill these NPCs - in which case you're justified in having future NPCs fight to the death - but still, give it a shot. Today I want to talk about it.
Enemies that run away do two things: one, it shows that the enemy is intelligent enough to recognize a fight gone bad and values its life, and two gives a chance for intel on the PCs to make it to other NPCs in the area.
The first part is the stronger one. Nothing makes a world feel more real than enemies that want to live enough they'll flee combat and deny PCs their loot/xp (assuming you do it that way.) But the second is also good, and can be a way to both upscale the difficulty for the PCs, and show them how controlling lanes of retreat can also be important. After all, that enemy that got away can show up with more enemies later, and unlike other groups of enemies, this one knows exactly who the healer and damage dealers are.
One thing I love about having NPCs surrender is that it gives you an opportunity to see who your PCs really are. How the PCs treat a surrendered NPC can tell you a lot about them. In fact, in my D&D game this became a key part of inter-party conflict when one PC wanted to use the surrendered NPC as a pack mule, and another refused to allow the prisoner to be mistreated, but was ok if the PCs wanted to pay the prisoner for services as a pack mule while they went to town.
In the end, the NPC wasn't mistreated and was turned over to a local magistrate. That NPC may not ever be able to repay the kindness, but that magistrate knows what happened and word will likely spread.
In other games though, surrendered NPCs can be a challenge all their own. For an example of this watch Saving Private Ryan. The main characters take a prisoner and while the efficient action is to shoot the guy, they choose to take the high road. Later there are consequences of this action, but I'll let you watch the movie to see if they're good or bad.
Fighting To The Death Is More Meaningful
The last point I'm going to make is this: NPCs who fight to the death will stand out a lot more when the PCs know other NPCs will surrender or run away. This is especially true if it is the same enemy type. After all, if normally goblins run away once their number is cut in half, what does it mean when suddenly the goblins are fighting to the last man? How can the PCs use that? What warning can it give?