只要留根 Always Leave A Root
八戒笑道：「師兄，你凡幹事，只要留根。既見妖精，如何不打殺他，卻就去解師父？他如今縱然藏羞不出，到晚間必定出來。他家裡還有舊衣服，穿上一套，來趕我們。八戒笑道：「师兄，你凡干事，只要留根。既见妖精，如何不打杀他，却就去解师父？他如今纵然藏羞不出，到晚间必定出来。他家里还有旧衣服，穿上一套，来赶我们。
逐句解 Line by line
Eight Rules laughed and said, "Elder brother, whenever you do a thing you leave a root behind."
留根 — leaving the root in. The image is agricultural and it is the opposite of the phrase he is about to reach for at the end of the argument. He has the whole speech planned — the matching phrase lands a hundred and thirty characters later.
"You saw the monsters — why did you not beat them to death, instead of going straight off to untie the master?"
既 sets up the reproach: given that you saw them. It is the standard way of building an accusation out of an admitted fact.
"Even if they hide their shame now and stay in, they are certain to come out by nightfall."
藏 is cáng, to conceal — not zàng, the storehouse reading in the master's name. 間 is jiān, marking the span of evening. He is doing something Eight Rules rarely does: thinking about later.
"They will have old clothes at home. They will put a set on and come after us."
Sixteen characters of straightforward operational reasoning. The clothes theft bought a few hours, not a victory, and Eight Rules is the one who notices.
Eight Rules argues for killing them, and — this is the uncomfortable part — his argument is better than Pilgrim's.
He points out that the clothes theft has bought a delay and nothing more. Seven creatures who cannot come out of a pool today will have spare clothes at home tonight, and they know which road the party is on. That is correct, and it is the kind of thinking Eight Rules is almost never given.
The phrase he opens with is worth keeping. Leaving the root in is what you do when you weed carelessly and the thing grows back. He closes the argument three speeches later on the matching expression, and the whole thing is built to arrive there.