The might of a ruthless woman.

Thats right. 

The mother loved seeing her daughter suffer and that wasn't all, that woman even went to the mountain every day until the poor girl was eaten clean by the vultures. From her confession, Song Yan could see that the woman relished watching her daughter die slowly and as painfully as possible because it satisfied her rotten heart. 

Now that she knew how Xue Ping killed those three women, Song Yan could see the shadow of that woman in Xue Ping. 

Moreover, this was a good way to deal with corpses. If they were ruined to the point that no one could even identify them, the case would be closed without being opened. What was more, Xue Ping chose the girls who had no one in their family. Yi Yu was an orphan and Gu Lao was born into a family that favoured boys over girls, there was no way they would look for her. 

Thus, she treated them carelessly as she wanted.