games usually either have real harsh punishments for going evil (in Bauldurs gate shop items are 200% more for a low reputation(evil)and random event spawns of powerful knights n stuff appear to wreck your team. while you get a half price discount on everything for the max reputation (good) and all the quests are like 'do the good thing for a sweet magic item, gold, experience and sweet stuff' or 'do the evil way for not much gold, hardly any xp, lose some reputation with the only payoff being the evil NPCs are better than the others, but your still better off getting a high reputation just under the 'evil guys will ditch you cap'.
However in a game like Fable or Fallout3, being good is slow and boring and being evil and slaughtering towns gets you tons of experience, items, allows you to buy their houses, gold, sometimes grows you demon horns and just all the sweetness.
Basicly i take the easy route