This discussion of the tree and its fruit is different from a very similar discussion in chapter seven. In chapter seven he uses this same image to keep us from being deceived by false prophets. Here it is about words and actions of all of us. He gives sobering words about accountability. . . for every idle word!
Nineveh repented at Jonah's preaching. The Queen of the South recognized Solomon's wisdom. They are interesting choices of judges. Jesus is greater than a lot of people. I wonder why he chose Jonah and Solomon?
When the unclean spirit is cast out and then returns, Jesus seems to be referring to the nation, not to an individual. The same is true with the judgment of Jonah and the Queen of the South. It has corporate, not individual.