Why is ammianus criticizing many of his countryman?

Ammianus frequently makes comments against the government, the army leaders, even his own emperor. However, these comments are often just recorded along with stories about military battles, without any criticism of the author except for the title "The Things Worth Knowing". It seems that he is criticizing these people because he wants to make sure that they would never do what they did again. But at least some of his countrymen might have taken their criticisms as unjustified personal attacks on them. So could it be that by criticizing his fellow Romans, Ammianus tried to protect himself from similar charges?

Rome’s great historian, Ammianus Marcellinus, chronicles many negative trends of his time, such as immorality and corruption, which has led to the rotted state of the society. The decay of Christianity also played a significant role in Rome’s faltered progress.

Explanation:

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a. favoring native born americans over immigrants.