埋怨馱魔 Grumbling Under the Burden
行不上三五里路，師父與沙僧下了山凹之中，行者卻望不見，心中埋怨道：「師父偌大年紀，再不曉得事體。這等遠路，就是空身子也還嫌手重，恨不得捽了，卻又教我馱著這個妖怪。莫說他是妖怪，就是好人，這們年紀，也死得著了。摜殺他罷，馱他怎的？」行不上三五里路，师父与沙僧下了山凹之中，行者却望不见，心中埋怨道：「师父偌大年纪，再不晓得事体。这等远路，就是空身子也还嫌手重，恨不得捽了，却又教我驮着这个妖怪。莫说他是妖怪，就是好人，这们年纪，也死得着了。掼杀他罢，驮他怎的？」
逐句解 Line by line
They had gone not three or five lǐ when the Master and Sha Monk dropped down into a mountain hollow, and Pilgrim, losing sight of them, grumbled to himself: "A man of the Master's great age, and still with no sense of how things stand.
山凹 (shān āo) is a dip in the hills, and dropping into one is enough to break the party in two. 埋怨 (mán yuàn), "to grumble"; 曉得 (xiǎo de), "to grasp," toneless 得; 事體 (shì tǐ), "how matters actually stand." The 里 here is the road measure, a Ming one running to something over half a kilometer -- and worth keeping apart from the 里 that ended the hamlet name earlier in the chapter, which is a unit of settlement, not of distance.
"On a long road like this, even empty-handed one would find one's arms a burden — I'd give anything to fling him off — and here he has me carrying this monster.
嫌手重 (xián shǒu zhòng), "to find one's own arms a drag," 重 read zhòng, "heavy." 恨不得 (hèn bù dé), "to be dying to," is still everyday Chinese for a wish you cannot act on -- except that Pilgrim is about to act on it. 捽 (zuó), "to fling down." 馱著 (tuó zhe) takes the 著 of an action still running.
"Never mind that he's a monster — even were he an honest man, at such an age he'd be as good as dead anyway.
莫說 (mò shuō), "never mind that." 死得著 (sǐ de zháo) stacks the toneless 得 of possibility onto the 著 of a thing achieved -- "would manage to die anyway." Watch the reasoning slide: from the weight, to the man's age, to the conclusion that killing him would hardly count.
"I'll just dash him dead — why carry him at all?"
摜殺 (guàn shā), "to dash down and kill" -- the same 摜 Sha Monk used in his joke about the three-pointed rock. 怎的 (zěn de), "why on earth," and the 罷 (ba) of a decision taken.
Left at the back with the fiend on his shoulders, Pilgrim talks himself toward murder by degrees. The grumble runs from the master's want of sense, to the sheer dead weight of the thing, to a piece of convenient reasoning -- even an honest man that old is as good as finished -- and lands on a decision. It is a small comic monologue of exasperation, and its menace is entirely real.
This is the hinge of the episode, and the irony is double. The one member of the party who saw through the disguise is about to be outmaneuvered, because his passenger has been listening to every word; and the master's compassion, by forcing Pilgrim to carry a monster he would gladly kill, has built the very trap that will separate them.