This is a problem on a lot of multiplayer games, not just Halo Wars. I agree it's not good sportsmanship though, if you go online you can't assume you will win, if your losing stick it out you may pull it back. I have been guilty of rage quitting in the past which I am ashamed of, tbh it was more about laggy connections due to the selected host rather than been beaten fairly. More games need to have penalties for quitting on multiple occasions or at least repopulate the game with another player if one leaves so people can enjoy their gaming experience.