I'm not sure what you mean by saying "But then I learned about the Mandate of Heaven." The "Mandate of Heaven" was a religio-political theory invented by the Zhou dynasty to justify its overthrow of the Shang dynasty, and later adopted by subsequent dynasties for their own political purposes (as well as by dissidents to support their own criticism of Chinese imperial regimes).

As a general rule, you should take all ancient history with a pinch of salt. Especially when it comes to last rulers of a dynasty, since as you guessed already, the new dynasty (who had a large influence on future writings), is often interested in depicting their predecessors in bad light.