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Of course, alignment is probably the single dumbest idea to ever come out of gaming, so let’s move on to the “morally gray” part of the question. Nearly every character I play could be considered morally gray to some extent. Characters who aren’t perfect are more interesting and fun to play (or watch or read about or whatever). That’s why you never hear the announcer guy on movie trailers say “Bruce Willis is a cop who follows proper police procedure to the letter and always flosses.”
Real people (and therefore believable characters) have a whole range of emotions, beliefs, cultural baggage, personal agendas, and psychoses that dictate their behavior, so when I play a character I try to make sure they act in accordance with those things rather than some abstract idea of “good” or “evil.” Even when people do things that they know are wrong, they usually justify their actions, at least to themselves. Sometimes a character I’m playing will do something that seems Just Plain Wrong to the other players, the GM, and even me personally, but to the character it’s something that Has To Be Done. In the best of cases, the character has made an active decision to do something that goes against his or her beliefs in order to prevent something even worse from happening.
Characters who are cartoonishly Good or Evil can work in a few very specific genres (pulp, space opera, some supers games, and a few others), but even those usually have at least a vague back story to explain why they act the way they do. Boy Scouts are a bit more versatile than Cackling Maniacs, but without some kind of motivation they’ll start to seem hokey and boring after more than a session or two. The only real exceptions are if you’re going for camp–where two-dimensional characters are kind of the point–or comedy, where it’s all for laughs anyway.