Why didn't MengMeng die in an evil ending?

Because Liu Meng cried out before she died, the two brothers and sisters relented and gave up the idea of killing her.

The City of Evil is a documentary-style horror film directed by Ma Kai and starring Dong, He Jia, Zhao, Wenzhong and He Yuanyuan. This film tells a series of stories about two college students who went to the countryside to shoot a documentary about fortune telling and followed the photographer to a remote villa.