The Scum Is Fascinated 

Xie Lan, Fan Wen, and Dong Zheng could understand what Fei Yin means. If the enemies would become leaderless, then the enemies would definitely seek blood from them cultivators, especially since they know that only Cheng Sheng could save their leader. This would mean that Cheng Sheng indirectly killed their leader. No wonder Cheng Sheng would choose to save the enemies' leader, but under a condition. 

After all, a leader is equal to being a father. Would you just watch on emotionlessly after your father was left to die? Definitely not! And so, they understand if the enemies would become more aggressive with their attacks if their leader would die. Not only will them cultivators have a harder time to fight the enemies, but this would also burden Cheng Sheng more since Cheng Sheng has to devise new plans again to suppress the enemies. Not to mention that it is his life that the enemies would go after this time.