It is always best done in an assertive way. To be honest, pointing the players to this question would not be a bad starting point. You could give examples from $media that you like and want the game to be similar to as well as $media that you do not wish to emulate.
For example: I want to run a game set in the Christian Kingdom of Jerusalem based on a historical setting where evil is what men do and neither pre-ordained by God nor Devil created. Paladins (Templar knights really) can be good and evil but mostly are just men. The films Arn: Knight Templar and El Cid would capture what I want well. Ridley Scott's aberration named Robin Hood is a summary of all things I do not want to see in the game...
As a side note, you could ask the players for advise on which system to use to best model what you wish the game to be. Because system apparently matters or something... In addition to that, the players could help design some of the NPCs that they will face thus creating NPCs they will care about -- or love to hate, or both!
You second part ([...]the most natural choice of play-style[...]) is trivial once you get player buy-in into your game. Clearly, your game must support a setting that conforms to your own rules as given above. In my setting above, killing large amount of Muslims bandits as Templar knights is clearly a very foolish thing to do. So is, as Muslims, killing Christian pilgrims. In this setting, the only thing violence will do is generate more violence. If all your in-games problems could be solved by klling it with fire, then as a GM you have failed. ^_~