Not a teacher, 53-year-old American.
I think you are asking about this sentence:
In my opinion, there is a problem with the structure of this sentence. The meaning is not clear as a result.He was a good ruler and that defying him would bring chaos to the country.
Perhaps this is the meaning:
"He was a good ruler, and defying him would bring chaos to the country."The implication of this sentence is that he - being a good ruler - was also a popular ruler. Defying a good and popular ruler would indeed bring chaos to the country. I really don't see how one could properly add 'that' to this sentence.
A more common use of 'that' is to indicate paraphrased information - as opposed to quoted information.
She said, "I am going to town." (A direct quote - these are the actual words she used.)
She told us that she was going to town. (We do not know the exact words 'she' used - only that she was going to town.)
I hope these examples help.