Are you a good guy or a bad guy?

In the westerns, it was always easy to spot the good guys and the bad guys. Good guys had white hats (don't ask me why, since they're so hard to keep clean) and bad guys had black hats. In a critique group, the line between good and bad guys gets hazy. What makes you either one? Are you a bad guy when you discover issues with a manuscript that you have to critique? Are you a good guy because you thought a story was perfectly written?

In a critique group, is telling all (and I do mean nicely telling all, with specific suggestions about how to improve the writing) a bad thing? Does it make you a bad guy with a black hat if you let the other writer know what you really think?