劈頭打死 One Blow To The Head
大眾一發上前，把缽盂揭起，果然見了本像，是一個六耳獼猴。孫大聖忍不住，掄起鐵棒，劈頭一下打死，至今絕此一種。如來不忍，道聲：「善哉！善哉！」大众一发上前，把钵盂揭起，果然见了本像，是一个六耳猕猴。孙大圣忍不住，抡起铁棒，劈头一下打死，至今绝此一种。如来不忍，道声：「善哉！善哉！」
逐句解 Line by line
The assembly came forward all together and lifted the alms bowl up; and sure enough they saw the original form — a Six-Eared Macaque.
一發, all at once, all together. 果然見了本像 answers 大眾不知 in the passage before: they can see it now, once it has been shown to them.
The Great Sage Sun could not contain himself. He swung up his iron rod and with one blow split his head and killed him — and from that day to this the kind has been extinct.
The chapter began with Wukong expelled for killing when he had been told not to, and it ends with him doing it again in front of the Buddha. In 劈頭一下 the word boundary falls between 劈頭 and 一下, so the 一 behaves normally rather than as part of a set phrase. 一種 takes zhǒng, a species.
The Tathāgata could not bear it, and said: "Alas! Alas!"
善哉 is the Sanskrit sādhu, and it ordinarily means "it is well" or "well said" — a word of approval. Here it is spoken by a Buddha who 不忍, could not bear what he had just watched, and the doubled cry carries grief instead.
Wukong kills him, and he kills him having been told not to a hundred characters earlier. 悟空休動手，待我與你擒他 was explicit, and 忍不住 — could not contain himself — is the narration's whole account of why he ignored it. The chapter opened with him expelled for killing when he had been told not to, and closes with him doing it again in front of the Buddha.
至今絕此一種 is the sentence that gives the act its weight. Not one creature but a kind — one of the four monkeys outside the ten classes, named by the Tathāgata five passages ago as a species — is ended with a single blow. The taxonomy that identified him also measured what was destroyed.
善哉！善哉！ is the Buddha's response, and the doubling is grief rather than praise: 如來不忍, he could not bear it. The chapter has spent nearly five thousand characters establishing that the false Pilgrim was a real being with a place in the order of things, however far outside the ten classes, and the one figure who understood what he was is the only one who mourns him.