Killing one's opponent in the Ten Year Bout is a taboo and must be avoided or else the faction that commits the killing, unless it is considered as an accident or unforeseen will be destroyed by the Origin World.
The people of the Central Regions view such a concept as normal. The future of the realm lies on the prospects of the numerous geniuses and in an event supervised by the Origin World, the concept of death must not be met.
They can kill one another for personal grudges when they are outside of the borders of the Sacred Lands. But when they are in the midst of the Red Lands and is in participating in the Ten Year Bout or the Century Recruitment no one is allowed for death to occur.
And with that in mind, the poison Xie Liu had used was something of a great shock to many. However, since no one from the proctors stepped forward to intervene, there was a public understanding that the poison of Xie Liu was non-lethal.