It seems that doubtful matters should not always be interpreted in the more favorable way.
1. For judgment seems to be more about that which happens for the most part. But for the most part it happens that people act badly, since “the number of fools is infinite,” as is said in Ecclesiastes 1:15 (Vulgate); for “the imagination of man's heart is evil from his youth,” as is said in Gen 8:21. Therefore we should interpret doubtful things rather on the side of the bad than on the side of the good.
2. Further, Augustine says that he lives piously and righteously who is an honest judger of things, turning to neither side. But he who interprets favorably that which is doubtful, turns to one side. Therefore one should not do this.
3. Further, man should love his neighbor as himself. But concerning himself man should interpret doubtful matters in the unfavorable way, according to Job 9:28, “I feared all my works.” Therefore it seems that things that are doubtful concerning our neighbors should be interpreted in the unfavorable way.
But against this, on Romans 14:3 “let not him who abstains judge him who eats,” the Gloss says, “doubtful matters are to be interpreted in the more favorable way.”
I respond, it should be said that as was said, when someone has a bad opinion of another person without sufficient cause, he thereby does injury to him and despises him. But no one should despise another, or do injury to another without a cause requiring it. And therefore when manifest judgments are not evident about someone's badness, we should consider him as good, interpreting in the more favorable way that which is doubtful.
To the first, therefore, it should be said that it can happen that he who interprets something favorably, errs more frequently. But it is better that someone err frequently, having a good opinion about a bad man, than that he err more rarely, having a bad opinion about a good man, since thereby injury is done him, while in the first case it is not.
To the second it should be said that it is one thing to judge about things, and another thing to judge about men. For in the judgment by which we judge about things one does not attend to good or evil on the side of the thing about which we judge, since it is not harmed however we judge about it, but one attends only to the good of the one who judges, if he judge truly, or evil, if he judge falsely, since the truth is the good of the intellect, while falsity is its evil, as is said in Ethics VI. And therefore each person should strive to judge about things as they are. But in the judgment by which we judge about men, good and evil is considered especially on the part of him about whim we judge; by being judged good he is considered more honorable, and more contemptible if he is judged bad. And therefore in such judgments we should rather tend to judge a man to be good, unless a manifest reason to the contrary is evident. And to the man who judges, a false judgment by which he judges well about another person does not pertain to the evil of his intellect, as neither does it pertain to its perfection to know the truth of singular contingent things, yet such a judgment does pertain to good affection.
To the third it should be said that one can in two ways interpret something unfavorably or favorably. In one way, by way of a certain supposition. And thus, since we should supply a remedy for bad things, whether they pertain to us or to others, it is helpful for the sake of more surely applying a remedy, that we suppose that which is worse, since a remedy which is efficacious against a greater evil, is much more efficacious against a lesser evil. In another way we interpret something as good or evil by way of defining or determining. And thus in the judgment of things one should strive to interpret each thing as it is, while in the judgment of persons, one should interpret more favorably, as was said.
Blogpost with some comments on this text of Aquinas on favorable judgment