He is a bad guy. He is not a good guy. Do the above sentences... have any iota of difference to their literal meaning?

Yes, there is a difference. When you say, "He is a bad guy", you have already decided that "he" is "bad". But when you say, "He is not a good guy", you are not defining him as "bad" but simply "not good". If that seems confusing, consider this statement: "I don't dislike parsley. But I don't like it either." Here, the person says he doesn't like parsley, but at the same time, he doesn't care enough about it to put it under his "dislikes" either; he simply can't be bothered with it and prefers to avoid it but can tolerate it to an extent. Hence, "He is a bad guy." is definitive and certain sentence. Whereas, "He is not a good guy." implies that this person may have qualities that could be considered deviant from the norm of what someone considers "good", or that he is not someone you can depend on or trust (or all of this). But at the same time, the speaker here is not stating that this person is a "bad" person, just that you shouldn't hold him up to the standards of what the speaker considers "good". The main thing here is that personalities aren't black and white. So in the first sentence, the speaker is stating that the guy is firmly in the "black" or "bad" category. But in the second, the speaker is only stating that the doesn't fall into the "white" or "good" category; rather, this person may fall into "grey" or "black" (though most likely "grey", since the speaker has yet to reach a point where he will definitely call him a "bad guy"). Here's another example/explanation as well: In the first sentence, the guy could be considered a "villain" in a sense if you take it in terms of a story. But in the second, the guy is simply Not a "hero", but he isn't quite a villain either. Think of it this way: In a story, there are all sorts of characters. So can you really categorize each of those characters easily as "hero" or "villain"? You will most likely find characters who might fall into both categories, and those who don't fall into either category. And here, the one that isn't in the "hero" category falls into "not a good guy" or "not a bad guy", depending on their qualities and roles. Hope this helps. read less

Grammatically and semantically speaking, the first sentence uses the adjective "bad" to refer to a person who is bad. Period. There is no doubt in the speaker's mind about the character/personality of the party being spoken about. The second sentence leaves an iota of doubt lingering in the mind of the listener about what the speaker really meant. read less