Since there were countless poisons here, and he would inevitably have to come into personal contact with them when refining poison formulas later on, testing the limits of his endurance beforehand was absolutely necessary.
Furthermore, this action was specifically directed at the Ten Thousand Poisonous Sacred Silkworm. It was meant for him to see. If Xu Yang wanted the Ten Thousand Poisonous Sacred Silkworm to acknowledge him as his master, he had to make him feel shock and fear. Being able to withstand Jia Xiu's poisonous blood, combined with so many astonishing methods, meant that Xu Yang wouldn't have to fear the venom of the Ten Thousand Poisonous Sacred Silkworm either.
Trapping him here would sooner or later drive him to despair. Whether or not Xu Yang was qualified to be his master wasn't up to the Ten Thousand Poisonous Sacred Silkworm to decide.
