

Sure, he's working for the greater good, but the only way he can get there is by knocking heads together until people listen to him. Lastly, think about the fighter's general problem-solving strategy: wading through hordes of enemies. Thieves do the robbing themselves, and Fighters probably didn't even notice it happening. Which class (Paladin aside) is goody-two-shoes enough to try to solve the bank robbery in QFG5? Magic users.

It's just a place for fighters to go and be fighters. It doesn't promote a cause, but it's not a front for any illegal operations, either. The poor fighter is too thick-skulled to nuance between the greater good (that's what Paladins are for!) and his own interests (he's not good at doing so while being heroic). The Thief obviously is consistent in acting in his own best interests. The MU is intimately connected with Erana, who we all know is a major force of good in the world. However, the Magic User is the "good" class, the Fighter is the "neutral" class, and the Thief is the "evil" class. However, some are more heroic than others. Every PC class has an implicit alignment.
