Unfortunately, in my experience, when someone approaches you about an RPG that you're playing, and they aren't a part of your normal group, there's only three reasons:
  1. They are a storytelling aficionado and want to build a cooperative narrative with other like-minded individuals (<1%. If this turns out to be the case, rejoice!)
  2. They want to experience what the heck an "RPG" is. (the grand majority. Of these, most will either find that the game is not to their liking, or will find their own fun by turning into option 3
  3. They want to have fun by killing things and taking their stuff (second part optional, sometimes).


You can try and talk it out with the player outside of the game, but the only way to avoid it entirely is to talk to the players in advance, tell them your planned tone for the game, and ask them what they hope to get out of it. The players are the game, so know what to expect!