The temptation against which the teacher seeks to guard his disciple is that of joining a band of highway robbers. The "vain men" who gathered around Jephthah Judges 11:3, the lawless or discontented who came to David in Adullam 1-Samuel 22:2, the bands of robbers who infested every part of the country in the period of the New Testament, and against whom every Roman governor had to wage incessant war, show how deeply rooted the evil was in Palestine. Compare the Psalm 10:7, note; Psalm 10:10 note.
Without cause - Better, in vain; most modern commentators join the words with "innocent," and interpret them after Job 1:9. The evil-doers deride their victims as being righteous "in vain." They get nothing by it. It does them no good.

If they say, Come with us - From all accounts, this is precisely the way in which the workers of iniquity form their partisans, and constitute their marauding societies to the present day.
Let us lay wait for blood - Let us rob and murder.
Let us lurk privily - Let us lie in ambush for our prey.

If they say, Come with us, let us lay wait for (k) blood, let us lurk privily for the innocent without cause:
(k) He speaks not only of the shedding of blood with hand, but of all crafty practises which tend to the detriment of our neighbour.

If they say, come with us,.... Leave your father's house, and the business of life in which you are; make one of us, and become a member of our society, and go along with us upon the highway;
let us lay wait for blood; lie in ambush under some hedge or another, waiting till a rich traveller comes up and passes that way, and then rise and shed his blood in order to get his money; and the same word signifies both "blood" and "money", and wait is laid for one for the sake of the other;
let us lurk privily for the innocent without cause; or "let us hide" (q), the Vulgate Latin version adds "snares"; so Vatablus and others, as the fowler does for birds; or "let us hide ourselves" (r); in some private place, waiting "for the innocent", the harmless traveller, who has done no injury to any man's person or property; thinks himself safe, and is not aware of any design upon him; going about his lawful business, and having done nothing to provoke such miscreants to attempt his life or take away his property: and which they do "without cause" as to him; "freely" (s) as to themselves; and "with impunity" (t), as they promise themselves and one another; all which senses the word used will bear.
(q) "abscondamus", Michaelis. (r) "Abscondamus nos", Pagninus, Montanus, Junius & Tremellius, Piscator; "abscondamus nosmetipsos", Baynus. (s) "gratis", Pagninus, Montanus, Michaelis, Schultens. (t) "Impune", Junius & Tremellius, Amama.

Murder and robbery are given as specific illustrations.
lay wait . . . lurk privily--express an effort and hope for successful concealment.
swallow . . . grave--utterly destroy the victim and traces of the crime (Numbers 16:33; Psalm 55:15). Abundant rewards of villainy are promised as the fruits of this easy and safe course.

Of the number of wicked men who gain associates to their palliation and strengthening, they are adduced as an example whom covetousness leads to murder.
11 If they say, "Go with us, we will lurk for blood,
Lie in wait for the innocent without cause;
12 Like the pit we will swallow them alive
And in perfect soundness like them that go down to the grave.
13 We find all manner of precious treasure,
Fill our houses with spoil.
14 Thou shalt cast thy lot amongst us,
We all have only one purse."

The verb ארב signifies nectere, to bind fast (from רב, close, compact), (see under Isaiah 25:11), and particularly (but so that it bears in itself its object without ellipse) insidias nectere = insidiari. Regarding לדם Fleischer remarks: "Either elliptically for לשׁפּך־דּם (Jewish interp.), or, as the parallelism and the usage of the language of this book rather recommend, per synecd. for: for a man, with particular reference to his blood to be poured out (cf. our saying 'ein junges Blut,' a young blood = a youth, with the underlying conception of the blood giving colour to the body as shining through it, or giving to it life and strength), as Psalm 94:21." As in post-biblical Hebrews. בּשׂר ודם (or inverted, αἱμα καὶ σάρξ, Hebrews 2:14), used of men as such, is not so used in the O.T., yet דּם, like נפשׁ, is sometimes used synecdochically for the person, but never with reference to the blood as an essentially constituent part of corporealness, but always with reference to violent putting to death, which separates the blood from the body (cf. my System der bibl. Psychologie, p. 242). Here לדם is explained by לדמים, with which it is interchanged, Micah 7:2 : let us lurk for blood (to be poured out). The verb צפן is never, like טמן (to conceal), connected with חבלים, מוקשׁים ,ח, פּח, רשׁת - thus none of these words is here to be supplied; the idea of gaining over one expressed in the organic root צף (whence צפּה, diducendo obducere) has passed over into that of restraining oneself, watching, lurking, hence צפן (cog. Aram. כּמן) in the sense of speculari, insidiari, interchanges with צפה (to spy), (cf. Psalm 10:8; Psalm 56:7 with Psalm 37:32). The adv. חנּם (an old accus. from חן) properly means in a gracious manner, as a free gift (δωρεάν, gratis = gratiis), and accordingly, without reward, also without cause, which frequently = without guilt; but it never signifies sine effectu qui noceat, i.e., with impunity (Lwenst.). We have thus either to connect together נקי חנּם "innocent in vain" (as איבי חנּם, my enemies without a cause, Lamentations 3:52): his innocence helps him nothing whom God protects not against us notwithstanding his innocence (Schultens, Bertheau, Elster, and others); or connect חנם with the verb (lie in wait for), for which Hitzig, after the lxx, Syr., Rashi,
(Note: Rashi, i.e., Rabbi Salomo Isaaki, of Troyes, died a.d. 1105. Ralbag, i.e., Rabbi Levi ben Gershon, usually referred to by Christian writers as Master Leo de Bannolis, or Gersonides, a native of Banolas near Gerona, died about 1342.)
Ralbag, Immanuel, rightly decides in view of 1-Samuel 19:5; 1-Samuel 25:31; cf. also Job 9:17, where the succession of the accents is the same (Tarcha transmuted from Mugrash). Frequently there are combined together in his חנם (cf. Isaiah 28:14.), that which the author thinks, and that which those whom he introduces as speaking think.