Maybe you’re the type who thinks that GMs don’t need to be evil. You’re probably in the minority. Most folks (players and GMs) assume that the guy or gal running the mission is evil - to the core. Some of us even like being thought of in that way.
So here’s a trick you can use to be more evil: Multiple Bad Guys
Here’s how it works: So the party is hired by the local merchant cartel to clear the bandits or orcs out of their mine. Seems the bandits attacked the mine and are now living there, with all the gold ore that the miners had dug out. Kind of standard, right? So they go out there and take care of those ruffians, right? Not so fast. Maybe as they approach the mine, there is a major battle between the bandits and the party. Seems like the bandits weren’t living in the mine, but in a camp just outside of it. But wait, there’s more. Inside the mine is a clan of dwarves who claim that the mine was always theirs and the humans were messing around in their ancestral home. They chased the bandits out, but the bandits had them in a state of siege. Nope, more to come. There is also a band of adventurers from a rival merchant cartel who have been tasked with getting as much ore and gold out of the mine as they can over the next three days, theoretically before the party gets here. It’s just that the party got here a little too quick and now the two adventuring parties are going to clash. You show up thinking you’re up against bandits, and find out you are fighting three separate groups.
This example is simplistic and unimportant. The thing you need to consider is throwing more at your players then you told them to expect. The local military/police is always a good one, because there are consequences to killing them. Anything worth hiring someone to retrieve is worth some bandit group trying to steal it first or from the party. There is no level of civilization that does not have some group claiming to own what is currently someone else’s due to an ancient grievance. There just are bad folks in the world who like to take what they can (and these are often called adventurers). Easy is boring!
There is another twist to this! What happens if the party learns that the local military, the local bandits, and their known enemy are all after the same thing? Can they team up? As a GM you should be ready for this, maybe even making it too difficult for them to succeed if they don’t. What happens then? Split the spoils or fight your recent ally. This is where it really starts to get fun!