Why was Hong Chengchou the biggest traitor in the Ming Dynasty?

It is true that Hong Chengchou was the biggest traitor in the Ming Dynasty! He used to be an important minister of the Ming dynasty, but later he became a loyal minister of the Qing army and a pioneer in destroying the Ming dynasty!

Hong Chengchou (1593- 1665), a native of Nan 'an, Fujian, was a Wanli scholar in the Ming Dynasty. Chongzhen was then the minister of war, the military governor of Henan, Shanxi, Sichuan and Hunan, and later transferred to Hebei and Liaoning to resist the Qing soldiers. In the fourteenth year of Chongzhen (164 1), Hong Chengchou led an army of130,000 men, which fought with the Qing army in Songshan, and was defeated, captured and cleared. In the first year of Qing Shunzhi, Hong Chengchou entered the customs with the Qing army and led the Qing army to suppress the anti-Qing rebels many times, killing Huang Daozhou and Xia Wanchun.

Interpretation of Ci Hai