Caveat lector: I am not British. EU-person still (regardless of Brexit... that's still not a true Brexit).
To vanilla D&D, I'll say it's Witcher with all its good and bad stuff.
The world is so full of dirt you can't help adopting stoic attitude toward evil, discrimination, poverty, corruption, prejudice and superstition.
The world is full of dirt, yet there are a lot of good people. Simple do-gooders, enlightened idealists and rich philanthropists. There are also decent folk who mind their own business, who prefer not to involve themselves in risky stuff, but who don't mind giving a hand now and than.
The world is full of dirt mostly due to overwhelming number of small-minded folk, who take every opportunity to vent their ire, their complexes on those who, in their narrowminded opinion, deserve it. They are easily swayed by power hungry demagogues and controlled by cynic power mongers.
The world is dirty because there is not enough education, because when you fight for survival on a battlefield, you don't get to question your orders.
There is also a serious crisis of authority, religion and bureaucracy - you don't trust your officials, period.
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Let's just say that in most of my games, be it fantasy, or not, there is a lot of this. Eberron comes close, though.
So to summarize, you kill orcs not because they are evil, but because, like Mongols, they are out to take away your livelihood and let you starve (if you're lucky). And you don't pardon orcs, because if they are let go free, they will resort to their ways.