Numion said:
Detect Evil, ping, wail on the bad guy. Let gods sort it out. Rinse. Repeat.
This is problematic on a number of levels:
- What do you do about people who detect as evil, but are still pretty much law-abiding (out of fear of punishment, if nothing else)? Ratting them out will likely ostrachize them, and might push them to lash out against society.
- Are the PCs able to convince others of the matter?
- If they are able to convince others, what should be done about such people? Just because you've outed the leader of the local Merchant's Guild as Evil it doesn't mean that there will be much in the way of consequences - especially if he has been an effective leader of the Guild and pandered to the interests of its members (who might be willing to ignore the negative consequences of his activities as long as they will get richer along the way).
- What about non-Evil people with plans which still may have nasty consequences for others?
- And let's not forget that magic items that conceal one's alignment aren't
that difficult to get, so any major villain with reasons to conceal his alignment will likely have one.
Mind you, Detect Evil is still a pretty useful spell - if the PCs detect an influential individual as evil then that's certainly reason to keep an eye on that person. But that doesn't mean that the PCs can avoid all work with it.
And anyway, being Good should be about more than just opposing Evil.