為人為徹 Do a Thing for a Man, Do It Through
那八戒見人散了，對行者道：「我們家去罷。」行者道：「你家在那裡？」八戒道：「往老陳家睡覺去。」行者道：「獃子又亂談了。既允了他，須與他了這願心才是哩。」八戒道：「你倒不是獃子，反說我是獃子。只哄他耍耍便罷，怎麼就與他祭賽，當起真來？」行者道：「為人為徹。一定等那大王來吃了，才是個全始全終；不然，又教他降災貽害，反為不美。」那八戒见人散了，对行者道：「我们家去罢。」行者道：「你家在那里？」八戒道：「往老陈家睡觉去。」行者道：「呆子又乱谈了。既允了他，须与他了这愿心才是哩。」八戒道：「你倒不是呆子，反说我是呆子。只哄他耍耍便罢，怎么就与他祭赛，当起真来？」行者道：「为人为彻。一定等那大王来吃了，才是个全始全终；不然，又教他降灾贻害，反为不美。」
逐句解 Line by line
Now Eight Rules, seeing that the people had scattered, said to Pilgrim: 「Let's be off home.」
Deictic 那八戒: 那 = nà. 散了: 散 = sàn, the verb "to disperse." Sentence-final 罷 = ba, hortative.
Pilgrim said: 「And where is your home?」
Interrogative 那裡 = nǎ, "where" — the pre-differentiated form of 哪, and to be distinguished throughout this chapter from deictic nà.
Eight Rules said: 「Back to old Chen's place, to sleep.」
睡覺: 覺 = jiào, the sleep sense, against pypinyin's default jué. An audit-invisible reading — the token must be checked directly.
Pilgrim said: 「Idiot, you are talking nonsense again.」
獃子: suffixal 子 = zi. 亂談, to talk at random.
Since we have granted him his request, we must see this vow of his discharged for him — that is the right way of it.」
Two readings of 了 in one segment, so positional fixspecs are required: 既允了他 is aspectual le; 了這願心 is resultative liǎo, "to settle, to bring to completion" — to discharge a vow. Sentence-final 哩 = li by GLOBAL_PY.
Eight Rules said: 「It is you who are the idiot, not I, and yet you call me one.
Adversative 倒 = dào, "on the contrary" — distinct from dǎo, "to topple." Suffixal 子 = zi twice. 不是: mechanical sandhi gives bú before fourth-tone 是.
We had only to fool him a while for sport and let it go at that. Why go through with the sacrifice for him and take the whole thing in earnest?」
便罷: 罷 = bà, the verb "and have done" — predicative, not the sentence-final particle. 當起真來: 當 = dàng, "to take as, to regard as" — the 當真 sense.
Pilgrim said: 「If you do a thing for a man, do it through.
為人為徹: both 為 = wèi, the purposive "for" — "acting for someone, act for them to the end." A proverb, matched in the novel by 救人救到底.
Let that Great King come and eat first, and then it is a whole beginning and a whole end;
Deictic 那大王 = nà. 一定: mechanical sandhi gives yí before fourth-tone 定. 全始全終, the thing seen through from end to end.
otherwise we should only have him send down calamity and leave misery behind him, and it would turn out badly.」
教 = jiào, causative "to have, to cause." 降災: 降 = jiàng, "to send down" — to be kept apart from xiáng, "to subdue," which this chapter also uses. 反為不美: 為 = wéi, the verb "to turn out, to become."
The exchange is a small masterpiece of characterisation by argument. Eight Rules, having played his part in the deception, considers the job finished the moment the audience has gone home; the vow was a means, the villagers are satisfied, and there is a warm bed at the Chen house. Pilgrim's objection is not moral squeamishness but a professional's sense of an unfinished operation. The demon has not been drawn out. Until he is, the village remains exactly as it was.
為人為徹 — do a thing for a man and do it through — is the chapter's operative maxim, and Wukong states it as a proverb rather than a decision, which is how he generally frames his own tenacity. The reasoning that follows is entirely practical: a half-performed sacrifice would leave the god insulted and free to retaliate, and 反為不美, the thing would turn out badly. That last phrase is one of the novel's characteristic understatements for catastrophe.
Worth noting how the register drops. Eight Rules calls Pilgrim 獃子 back at him — the epithet is normally Wukong's for the pig — and the retort has the flat playground shape of you're the idiot, not me. The novel keeps its two comic leads in constant low-grade friction even while the plot turns serious around them.