We must make one concession to the critics of rationality. A person or group wanting to be rational and scientific may be inclined to wishful thinking about it. He may believe that he can has a correct scientific theory when he doesn't. He may believe in the results of long chains of reasoning within a theory even when the results counter common sense and have immediate bad consequences. This may cause him to act worse than a person without a theory who may refrain from an action on the basis of its immediate and apparent bad consequences.
Proper rationality requires a certain modesty about one's theories.
Not to be coy about it, the theories that come to my my mind as having led to the most bad actions recently have been variants of Marxism, although religious theories have had similar bad results in the past. Thus the Inquisition sometimes used a theory of demonic posession which justified torturing a person to drive the devils out. Further, since any earthly tortures were infinitesimal compared to those of hell, forcing a confession and repentance by torture was doing the victim a favor. I don't mind if the reader who is so inclined, makes up his own examples of the bad consequences of other wrong theories based on the crimes that are most salient in his own mind. The number of hits on this page since 1996 April 20.