The accomplishment of the Qin was to reunify China under a single emperor for the first time in 500 years, ending the Warring States period. As you can imagine, the making of that particular omelet required the cracking of quite a few eggs, and the great Qin emperor Qin Shuangdi and his descendants developed a reputation for brutality that was justified. But it was also exaggerated for effect so that the successor dynasty, the Han, would look more legitimate in the eyes of heaven. So when recounting the fall of the Qin, historians focused on how a bunch of murderous eunuchs turned the Qin emperors into puppets, not literal puppets, although that would have been awesome. And these crazy eunuchs, like, tricked emperors into committing suicide when they started thinking for themselves, etc. So the Mandate of Heaven turned away from these suicidal puppet emperors, which set up a nice contrast with the early Han emperors, such as Wen, who came to power in 180 BCE and ruled benevolently, avoiding extravagance in personal behavior and ruling largely according to Confucian principles. Under Wen, there were no more harsh punishments for criticizing the government, executions declined, and most importantly, for the Confucian scholars who were writing the history, the government stopped burning books.