Legitimate grievances? Mostly it just means the sides have reasons for acting the way they do. Ancient feuds or acts of aggression that were never reconciled. People doing the things they have to do but it impacts another group of people negatively and the affected people don't look at or care about the reasons, just that they are being hurt.

For point 4, pit characters' morals against results and see what happens. Are they willing to sacrifice some for the good of many? Will they tolerate lesser evils to ward off greater ones? Things like that.

Like... say there is a fascist-type ruler who is the first line of defense against an army of orcs or undead or whatever. Do you let him get steamrolled to weaken the enemy and dispatch an evil, militant kingdom? What about the people living there?

I mean there's no end to possibilities, it just depends on the world and story you want to tell. The only thing is, make sure it's not completely cynical. Everyone is flawed, but that doesn't mean everyone is bad or there aren't good guys and bad guys. Game of Thrones is good inspiration here.

The first thing to understand about how to have NPCs with legitimate grievances be a thing in a TTRPG is to imagine the world as a real one and the NPCs as people within that world. If you're just thinking of the setting as a flat backdrop for gameplay and the NPCs as colorful questgivers whose protection is a thin excuse for the plot to happen, then you're not going to be thinking in terms of things like what needs various NPCs might have and why they're unfulfilled or violated.

I find it useful to imagine that I hold the same view as the PCs, and then interrogate myself on the question "what if I'm wrong?". Specifically, asking how I would be able to tell if I'm wrong (i.e. what signs might or might not exist that would help to set me right), or ways that my intended solution to a problem could have ripple effects beyond what I expect.

I'm of the view that in life, we never really solve problems, we just transmute them into different problems that are easier to manage under our particular circumstances. The new problems created by a solution aren't necessarily going to be evident immediately; sometimes it takes time for the sticking points of a solution to present themselves.

Smart NPCs are a big part of making this work. If the players are on one side of a political issue, and there's an NPC who is in direct opposition to them, then if that NPC is smart, they'll be thinking ahead about what the players are likely trying to do. The NPC might not be able to stop the PCs from implementing their solution of choice, but might reasonably be able to figure out how to subvert this solution for their own ends (or figure out how to reverse or overcome the obstacle that this solution may present for the NPC)