How do the NPCs know you're not to be trusted?

A lot of NPCs in my games react poorly to delvers in general.

How do NPCs who haven't met the delvers in particular know they are untrustworthy greedy violence-mongers?

It seems unfair from a player perspective that somehow people "know" you don't take prisoners, you don't tell nobody your business, and yet somehow people know not to surrender to you or run because they know you'll slit their throats or shoot them in the back. What gives? How do GMs justify this unfairness?

Here are four ways.

1) Word gets around. Let's face it, PCs are terrible, terrible, terrible at covering their tracks. They also want credit for all of the cool stuff they did, adulation for their special loot, and respect and fear of all around them. So they don't tend to do much besides destroy everything like rabid wolverines, sell stolen loot at a premium as a collection that fails to disguise its provenance, and so on. They also tend to either leave corpses around or blood splatters where their new zombies walked off from. So "we didn't tell anyone or leave survivors, how do they know?" You've left enough clues to make Inspector Clouseau look like Sherlock Holmes.

2) People jump to conclusions. They're stereotyping you. Why is that?

3) You're not the only one. If you're going around plundering and murdering and betraying and taking no prisoners except to question and later kill and zombie, others probably have done this. It's not like you invented being ruthless and greedy. So people assume you are. You have to prove you have Sense of Duty but they'll probably assume you have Greed and Bloodlust and a Quirk: Kills everyone who ever wronged him in any way and "wronged" means "inconvenienced."

4) Summon Spirit. The kicker. Divination. People can find out a staggering amount about you even if you kill all of your enemies. Their friends can use Crystal Gazing to look in on you. They can summon their dead buddies and talk to them, even if you destroyed the corpses. They can use History spells to check out where that goblet you won't discuss came from. They can just ask and get a solidly accurate answer.

So really, being a murderous delver is fine . . . but it's hard to conceal. Foes will assume this is you, and foes with any knowledge can find out what you're like if they hear you are coming. And even random NPCs in town curious about the loot you sold them can find out very solid details on how you found it, with some access to magic. Pretending otherwise is failing to recognize that a fantasy world with common divination magic means just doing something is probably enouugh for someone with enough cleverness and will to find out what you did.

That's why the NPCs don't trust you quite as quickly as you'd hope!