After Leng San had possessed the Sky-Devouring Viper King... Leng San started to understand the relationship, including the difference between the Viper King and other beasts like Mr. White and Mr. Black...
Partly, it was because the Viper King was only a fragment of the main body!! There was not even a physical body, only the spirit of the main conscience that was eager to escape from the tower...
This prevented Leng San from calling the Viper King to come out to fight like Mr. White and Mr. Black... But Leng San could release the spirit of the Viper King in a form similar to the fire-goddess form of Freya... It was also considered dangerous because the Viper King could resist Leng San's orders freely when he came out...
The only way for Leng San to control the Viper King was to release and recall him!! With the power of the Beast Spirit Sign tied to the spirit of the Viper King, the Viper King could not refuse if Leng San pulled him back into the seal tattoo...