He who interprets doubtful matters for the best, may happen to be deceived more often than not; yet it is better to err frequently through thinking well of a wicked man, than to err less frequently through having an evil opinion of a good man, because in the latter case an injury is inflicted, but not in the former.
St. Thomas goes on to distinguish being wrong about things and being wrong about people. The former is worse for you than the latter, since being wrong about an evil person reflects more on the person than on your own intellect.