後人之害 A Plague To Later Men
二郎與六聖道：「不趕他倒也罷了，只是遺這種類在世，必為後人之害。」至今有個九頭蟲滴血，是遺種也。二郎与六圣道：「不赶他倒也罢了，只是遗这种类在世，必为后人之害。」至今有个九头虫滴血，是遗种也。
逐句解 Line by line
Erlang and the Six Sages said, "Letting him go may do well enough; only, leaving such a breed alive in the world is bound to be a plague to the men who come after."
為 is wéi here, the copular — to be, to become — where pypinyin gives the purposive wèi. 罷了 takes 罷 at bà by the registered pairing, and the spec matches that ruling rather than overriding it. 種 is zhǒng, a breed or kind.
To this day there is a nine-headed creature that drips blood; it is of that surviving stock.
血 is xuè here, the ordinary reading — a hundred and eighteen graphs after 血淋淋 in the same block took xiě by registration, which is as clean a demonstration of that ruling as the chapter offers. 種 is zhǒng again.
The Six Sages get the last word on the fight, and it is a criticism. Letting the creature go may be defensible, but the breed survives, and a surviving breed will trouble people later. They are not arguing with Pilgrim so much as recording a reservation.
Then the narrator steps out of the story altogether. 至今 — to this day — moves the whole account from the Tang dynasty to the present tense of whoever is telling it, and offers the reader an explanation for something outside the book: there is a nine-headed thing that drips blood, and this is where it came from.
It is the only aetiology in the chapter, and it is placed with some care. The Sages predict that the breed will be a plague to men who come after; the narrator immediately confirms that it was. The prediction and its confirmation stand in adjacent sentences.