Good vs. evil thinking causes us to lower our value of a person's opinion, or dismiss it altogether, if we find out that person has behaved badly. We no longer wish to affiliate with those people and furthermore we feel epistemically justified in dismissing them.
Sometimes this tendency will lead us to intellectual mistakes.
Take Climategate. One response is: 1. "These people behaved dishonorably. I will lower my trust in their opinions."
Another response, not entirely out of the ballpark, is: 2. "These people behaved dishonorably. They must have thought this issue was really important, worth risking their scientific reputations for. I will revise upward my estimate of the seriousness of the problem."
I am not saying that #2 is correct, I am only saying that #2 deserves more than p = 0. Yet I have not seen anyone raise the possibility of #2. It very much goes against the grain of good vs. evil thinking: Who thinks in terms of: "They are evil, therefore they are more likely to be right."
(Which views or goals of yours would you behave dishonorably for? Are they all your least correct views or least important goals? With what probability? Might it include the survival of your children?)
I do understand that this line of reasoning can be abused: "The Nazis went to a lot of trouble, etc." The Bayesian point stands.
Another example of misleading good vs. evil thinking stems from the budget. Many people believe:
3. "If the Republicans win, they will irresponsibly cut taxes and do nothing real to control spending." You may have even seen this view in the blogosphere.
One response to this is 4. "We should ensure that the Republicans do not win and criticize them every chance possible."
An alternative response is 5. "Sooner or later the Republicans will in fact win and I cannot prevent that. Right now the Democrats should spend less money, given the truth of #3. In this regard the Republicans, although evil, are in fact correct in asking the Democrats to spend less money, if only to counterbalance their own depravity."
I do not see many people entertaining #5. #5 implies that a group judged as dishonest should be granted some probability of speaking the truth on an important issue. (Nor will pro-Republicans be attracted to a claim which portrays their group as dishonest.) Note also that by accepting #5 you are admitting and partially accepting the ability of the Republicans to "out-game" the Democrats. That makes #5 even harder to accept.
Again, I am not asking you to buy #2 and #5 outright. I am simply suggesting they have a higher "p" than many people are willing to grant them. And that is because we are accustomed to judging the truth of a claim by the moral status of the group making the claim.