The notion that Liu Bang extensively exterminated his early supporters is inaccurate, plus, Liu Bang isn’t really known for benevolence in Chinese history, he was better than Xiang Yu in that regard perhaps, but definitely not someone you would call “benevolent”. Rather, he was more known for his ruthless pragmatism, unbreakable perseverance, and that unique tinge of ruffianism, thanks to him being the first emperor of common origin.

Qin Shi Huang was branded a tyrant, because of a lot of reasons: He was anti-confucian, treated the scholars badly, anti-Qin propaganda of the Han era, resentment of people from former warring state kingdoms towards their countries being destroyed, imposing harsh Qin-style laws on people of other former kingdoms, engaging in extremely costly activities such as great wall construction, southern campaigns against the Baiyue, etc. etc.

Now let me explain why it’s inaccurate to say that Liu Bang extensively purged his old supporters. After Liu Bang’s victory, his most prominent supporters were made marquises. Among the more than a hundred marquises made, only very few were subjected to Liu’s “purge”. People like Xiao He, Cao Shen, Guan Yin, Zhang Liang were not purged. The victims of Liu Bang, with the sole exception of Chen Xi, who openly rebelled in 197 BC, were all vassal kings.

What are vassal kings? To understand this you have to understand that the Chu-Han contention was not all about Liu Bang and Xiang Yu. When the Qin dynasty was falling there were many rebels from all across the country, many of them ended up with their own armies, territories, and connections by the time Qin fell, in effect becoming semi-independent warlords. This was why both Xiang and Liu had to create separate kingdoms and grant it to people after their respective victories of Julu and Gaixia, not because they didn’t want a completely unified country under their total control, but because these warlords were powerful and you had to appease them by making them kings.

There is a difference between Liu and Xiang though. Xiang Yu’s assignment of kingdoms was an active attempt to shape the post-Qin “world” order, which failed miserably, whereas Liu Bang’s was passive—i.e. he was “forced” to make people kings because these people were already powerful warlords and he needed their help to defeat Xiang Yu. If you look closer, you’ll see that the vassal kings under Liu, with the exception of Han Xin, who carved out his own kingdom while serving under Liu Bang, were all warlords that were already very powerful before the Chu-Han contention. Peng Yue, for instance, was one of the first to rebel against the Qin state, in the years after he based himself in the lands of Liang, carved out his own state, and gathered a powerful army that only Xiang Yu himself could defeat. Ying Bu was a vassal king made by Xiang Yu, Xin the King of Han was heir to the reestablished traditional Han Kingdom, etc.

So, those vassals kings did not “serve” Liu Bang, rather, they were more like Liu Bang’s fellow allied kings (Liu Bang was also just the King of Hanzhong before he assumed imperial title). And as we know, often in history after eliminating their common rival, allies tend to start fighting each other. Liu Bang saw this, but he also knew that bringing the country under the emperor’s total control, like what Qin Shi Huang did, was not realistic at the time. Hence he came up with a unique solution—immediately after defeating Xiang Yu he initiated a process to remove the semi-independent vassal kings one by one and replace the kings with people from his own family. Hence you would have Liu Bang ruling the old Qin lands directly, while his family members rule the rest of China as vassals. It follows that Liu Bang’s purge of vassal kings was fundamentally different from, say, Zhu Yuanzhang’s purge of his followers, because it was simply a continuation of the Qin-Han civil war to unify the country.

Thereafter you see:

202 BC

-Xiang Yu was decisively defeated and killed in the battle of Gaixia, following which Liu Bang assumed the imperial title.

-Han army marched into the kingdom of Linjiang and defeated its king Gong Wei (Xiang Yu’s ally).

-Zang Tu the king of Yan (former Xiang Yu’s ally) rebelled and was quickly defeated. Lu Wan, Liu Bang’s childhood friend, was made the king of Yan in his place.

-Some kings were relocated from the lands they were formerly granted, in order to weaken them. Han Xin (韩信) was moved from the kingdom of Qi to the kingdom of Chu, while his former Qi kingdom was split among members of the Liu family. Xin the king of Han (韩王信, different person!) was moved from old Han lands to the northern border. Wu Rui the king of Hengshan was moved to the remote land of Changsha, where thanks to this remoteness he became the only vassal king that escaped Liu Bang’s purge.

-Tian Heng, the supposed legitimate ruler of Qi, committed suicide.

201 BC

-Liu Bang pretended to tour Han Xin’s land and arrested him by surprise, revoking his kingdom of Chu and splitting it among members of the Liu family.

200 BC

-Liu Bang suspected Xin the king of Han of collaboration with the Xiongnu, the later, in fear, defected to Xiongnu and started a rebellion. In the Xiongnu-Han war that followed Liu Bang got the worse of it, but Xin was successfully removed from his kingdom and a Liu family member replaced him.

198 BC

-Liu Bang arrested Zhang Ao the king of Zhao, on the pretext that he was planning to murder him, and revoked his kingdom, granting it to a member of Liu clan.

197 BC

-Chen Xi rebelled in the northern part of Zhao.

196 BC

-Chen Xi was defeated, Han Xin and Peng Yue the king of Liang were both charged with collaborating with Chen Xi, the former was executed while the later, again, was arrested by surprise, executed, and had his kingdom revoked and split up.

-Ying Bu the king of Huai Nan, seeing the ends of Han Xin and Peng Yue, rebelled. Liu Bang led an army against him in person and defeated him. His kingdom was split up.

195 BC

-Lu Wan the new king of Yan was charged with treason, and Liu Bang sent an army against him. Liu Bang died before his army got there, but Lu Wan abandoned his kingdom and fled to Xiongnu, his kingdom was given to a member of Liu clan.

Thus, by 195 BC, Liu Bang has accomplished his objective. Since his first rebellion against the Qin dynasty it took him 7 years to defeat Qin and Xiang Yu and become emperor, then he spent another 7 years to wipe out semi-independent warlords to achieve total control of the empire by his clan. This allowed the Han empire to avoid the fate of Qin and Xiang Yu’s Western Chu and successfully transition into China’s first long-lasting unified empire.