Fakename, you do come up with interesting questions!
It is not a sin to make a mistake. It could be a sin against prudence to fail in the duty to inform yourself or to fail to take reasonable care in your actions, which could lead to mistakes, but an honest mistake is not a sin, per se. Jesus could have made an honest mistake. Our Lady could have made an honest mistake. They probably did.
As for complimenting Hitler’s mustache, that may not be the best example, because his mustache is, by most standards, objectively dreadful. The same goes for his artistic ability. He was a mediocre artist. So if you compliment either of those, someone might wonder about your judgement.
But let’s say that, objectively speaking, he did have some virtue in his life. Let’s say that he never failed to look after his mother, that he sacrificed his own convenience, wealth, and safety to see that she was properly taken care of. That could be noted by a biographer, as an example, without commititing an offense against the truth, because it is no sin to note the virtue in even a great sinner’s life and to admit that it was, indeed, virtue. Again, a merciful heart such as one would have found in the Holy Family would be quick to advocate for a sinner to the extent that they had any true virtue they had in their lives.
What is an offense against the truth and what Our Lord would* not* do is to take the virtue in a sinner’s life and make it out to be an excuse for their sins. The Pharisees and scribes, for instance, did a lot right. They had teaching authority, because they knew the law and they taught it. What did Jesus say about them? “The scribes and the Pharisees have taken their seat on the chair of Moses. Therefore, do and observe all things whatsoever they tell you, but do not follow their example. For they preach but they do not practice.” Matt. 23:2-3 He made it clear it what respects they deserved respect, and in what respects they didn’t.
If you are confronted with Hitler’s concentration camps, you can’t defend his action by pointing out some great public work accomplished by the slave labor in the camps. That’s an offense against the truth. You couldn’t defend his military build-up, which he put into action as part of his intention to take over all of Europe by force, as if it were some beneficial public works project that revived German industry. Maybe it had that politically-beneficial side-effect for him, but that was not the primary intended end. Power for Hitler and the capacity to carry out his evil intentions was the intended end, and that is the truth. So if you compliment Hitler’s industrial build-up of Germany in a way that seeks to get around that truth, then you are committing an offense against the truth.
The most common time that we commit this sin–that is, to make a parade of someone’s virtues as if these were excuses for vice–is, unfortunately, in our own defense. When the truth about our sinfulness confronts us, we like to try to make it out that we “aren’t such a bad guy.”
That isn’t the point of examining our consciences! We don’t go to doctor’s, get diagnosed with cancer, and then object “but look at these abs! I don’t need surgery! I don’t need chemo! How can you make a diagnosis like that, against someone with core muscles like mine!”
It is often useful to look at sin in exactly the same way. Look at it as something that injures our relationship with God, that injures the Body of Christ, that injures ourselves and what we are meant to be. Looked at in that way, you can see when it is relevant to talk about mitigating factors that might reduce culpability, and so on, and when one has to say, “This is a deadly situation, and it must change to reach a situation consistent with eternal life.”
I think that looking at sin in terms of injury and in terms of life and death and the failure of systems necessary for life is often a more useful metaphor than the courtroom model. Either one is a metaphor, either one has its place, and what I have found is that one starts breaking down, the other one can often enlighten what the situation is.
This does not mean that many people won’t find any compliment of anyone they find evil to be profoundly offensive. Keep this in mind, though: What is an offense to human sensibilities isn’t automatically an offense against the sensibilities of God. Still, it is kind to avoid getting people stirred up for no reason. Take their weaknesses into account, and do not upset them, if it can be helped. That is a moral imperative.