They can; unless the DM (whose NPCs they are) has already decided something else about them e.g. that Eliza the kitchen maid is a spy for the local Thieves' guild who are hoping to, maybe not rob the PCs blind, but use the PCs' info to let them know where other tasty jobs might await.
And it's highly unlikely (as in, it won't happen) that the players will ever design anything into these QPCs* that works against thei own interests.
* - QPC = Quasi-Player Character.
And this is exactly the type of thing that they are telling DMs to avoid. You have your group and your group loves your style, Lanefan, but I DESPISE this sort of crap. You know, I have lived my entire life with very very people actively betraying me? Oh, I have plenty of terrible people in my life, but they are kind of obvious.
Yet, here we are in literally the first example I've presented to you, and instead of allowing an innocuous love story between two nobody peasants that only the Player really cares about, you are immediately looking for "how can I turn them against the PCs to work against their interests"
Why? It can't be because you need conflict and drama, because I am assured you have spent the last 50 years with plenty of that in your games. Why does EVERYTHING need to be potentially used to hurt and betray the players? Why can they trust NO ONE, ANYWHERE? Do you know how utterly, mind-numbingly
boring that is? When literally everything is against the PCs no matter where, what or why, it is so utterly stale and uninteresting.
That's not the issue. The issue is that all these NPCs aren't clones; they're each different and will - or should - each have different goals and reasons for becoming hirelings or recruits for the PCs...not all of which might agree with the PCs' own agenda.
Same as hiring for a small business - you'll get some employees who are above-and-beyond loyal and honest and others who will steal from you at any opportunity; with most falling somewhere between those two points.
No one said they were clones. Players CAN make NPCs who are different from each other, and may have unique goals and reasons for joining the PCs. That doesn't mean they need to steal from the PCs. That doesn't mean they need to spy on the PCs. That doesn't mean they need to unlock the gate and allow in the enemy assassins to kill the PCs. That doesn't mean they need to poison the PCs. That doesn't mean they need to be secret devil worshipers who are slaughtering the local village women and hiding their bodies in the PCs garden.
It can literally be as simple as they joined the PCs stronghold to make enough money to take care of their sickly mother. And no evil groups or eldritch demons end up pressuring them into evil acts, or anything of the sorts. They can just be a person that the PCs help by being good employers who they have a good relationship with, and occasionally talk to to check on the status of their sick mother.
And you don't need a stranglehold over every single NPC ever conceived, for that story to be told by your player.
