The greatest fault of the Qin dynasty was its ruler. Qin Shi Huang started as a power ruler that was respected by his people. He was a cruel and harsh leader but commanded the respect from the people he ruled over. When he first began to rule his approval was decent, but once he lost that approval from his people, he spiraled. As conditions worsened, people made attempts to assassinate Qin Shi Huang. One of his most prominent traits to lead to his downfall was his paranoia. It is believed that the Great Wall of China was built as a result of his paranoia.  There is evidence of invasions and enemies in the north, but it did not warrant the size and length of the wall created during this time period. Because of this constant fear of deception the emperor trusted no one. He didn't trust his guards, scholars, adversaries, or even his own family. It was speculated at the time that Qin Shi Huang killed off all of his siblings to prevent anyone from dethroning him. This was confirmed by evidence found in the terracotta army pits. In a small pit right next to the horse stables, archaeologists found the tombs of the emperor's siblings. After digging into one of the pits they found a skull with an arrow embedded into the back of it, suggesting that they were executed. . This confirmed the emperor's intense mistrust, and ultimately the cause of his downfall.
Knowing that he could not live forever, as he aged the emperor's main goal was to search for immortality. In his old age he believed that his empire was facing destruction from past history. To combat this, he believed that it was his responsibility as told by the gods to "cleanse" his country. He believed that to do this he had to burn all books that belonged to other beliefs, religions, and even past Chinese history.  This event was not well perceived and is now known as one of the most infamous events in Chinese history. The only books he decided to spare were books on medicine, pharmacy, and immortality in hopes that these books held the answers to live forever. After this he became even more obsessed with finding the key to immortality. He began to send scholars and officials on blind quests to discover the "Island of the Immortals." When they returned with nothing he concluded that they did find it but were just not telling him. To demonstrate his discontent, the emperor gathered 460 scholars and buried them alive.  This act of aggression marked the beginning of the end of the First Emperor. This quest for immortality is what eventually killed him. Although the exact cause of death is unknown, scholars and historians believe that it was mostly due to what he was ingesting. Towards the end of his life, he began getting desperate and had doctors create immortality potions. These pills and potions were found in the burial complex of the emperor and analysis concluded that they contained very high levels of mercury.  Long periods of ingesting mercury has two major affects on the body: kidney failure and brain damage.  While there are many speculations and theories on what actually killed him they are only conspiracies until researchers can find his tomb.
Many more mysteries about this time period can be uncovered once researchers find the emperor's final burial ground. The search for his tomb has continued for decades as archaeologists slowly continue to dig out the area. Until then historians can only speculate based off of what incredible things were found in the pits.