There are many reasons why the first emperor Qin Shihuang was scolded and vilified. First, before the Qin Dynasty, China was composed of independent vassal states, whose cultures and customs were actually different. Therefore, after Qin Shihuang unified the six countries, the people of these vassal States who were forced to unify were not very convinced. Many people hated Qin Shihuang because they were destroyed. Under the influence of this emotion, people's evaluation of Qin Shihuang can't be all positive.
The second reason is that the laws of the Qin Dynasty were harsh. Since Shang Yang's political reform, Qin has been ruling the country according to law, and this principle has been extended to the whole country after the unification of the six countries. Under the enforcement of strict criminal law, some people who have suffered great punishment because of carelessness will think that Qin Shihuang is a tyrant, so they will try their best to vilify him.
The third is because the corvee is heavy. Take the Great Wall, which plays an important defense function, as an example, its appearance has greatly protected the safety of the people in the Qin Dynasty. However, compared with this, people feel more that Qin Shihuang requisitioned active servicemen to build the Great Wall, which also caused many deaths. So, some people who were at a loss remembered to overthrow the rule of the Qin Dynasty.