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.