絕了後患 That Ends the Trouble Later
行者道：「花皮會吃老虎，如今又會變人。這頓打死，才絕了後患也。」長老謝之不盡，攀鞍上馬。行者道：「花皮会吃老虎，如今又会变人。这顿打死，才绝了后患也。」长老谢之不尽，攀鞍上马。
逐句解 Line by line
Pilgrim said, "A mottled coat is fit to eat tigers, and now it can turn into people as well.
花皮, mottled hide, stands for the leopard itself. There is an old belief behind the line: leopards were held to prey on tigers, so a leopard outranks the beast everybody else fears. Wukong is saying the creature was already dangerous before it learned shapeshifting.
Killing it this time is what ends the trouble that would have come after."
後患 is trouble in the future, the harm that comes later — still exactly the modern word for a problem stored up. The argument is prevention, and it is the closest anybody comes to justifying the fire.
The elder gave thanks past all telling, and climbed into the saddle.
謝之不盡, thanks without end, is the first of two places in this chapter where the tone of 不 depends on a character whose own reading is raised after the fact — the character for exhausting comes out higher than it starts, and the negative in front of it has to follow the finished value, not the first one.
Wukong's justification comes after the fire rather than before it, which is honest of the novel. He did not argue the case and then act; he acted and then explained.
Tripitaka's response to the destruction of a household is to thank his disciples and get on the horse. The book does not give him a word of objection, and given how freely he objects to Wukong killing bandits in other chapters, the silence is doing something.