TLDR: What believable real-world consequences can there be for killing orcs, goblins, etc. when such acts are not illegal nor necessarily frowned upon by one's society?
I am preparing a Dungeons and Dragons campaign and would like to treat humanoid races (orcs, goblins, etc.) differently than usual. In Dungeons and Dragons these races are evil by nature and as such are often slaughtered to the last whenever encountered by player characters playing the 'good' races (humans, elves, dwarves, halflings, gnomes). As a person I find this disagreeable and as a player or game-master I find it boring.
What reasons could there be in a fantasy world for racial and cultural divides to not result in chance encounters being combats to the death? Note that I am not asking for a world where everyone gets along. I just want to create a setting where initiating combat with and killing sentient creatures is a non-trivial decision. If that's what they decide to do I'll allow it but there should be consequences (consequences, not punishment).
Some ideas I've been thinking about. Willing to modify or discard:
- Humanoids are not evil, they are merely different. However, the 'good' races may still view them as evil.
- Pantheons are not grouped by race. Good gods are worshiped by good creatures regardless of race, same goes for evil gods. Worshipers don't necessarily get along but good gods may frown upon wanton slaughter even of evil creatures.
- Ecology. Wiping out a goblin nest may cause unexpected problems for nearby human settlements. Haven't figured out a concrete example for this that doesn't feel contrived.
- Budding ideas of rights in 'good' society. I don't like this idea very much. If this happens at all in the game world I want the player characters, not myself, to introduce it.
I didn't put this in RPG stackexchange because this is about how the game world will work rather than about setting player expectations.