They lay wait for their own blood - I believe it is the innocent who are spoken of here, for whose blood and lives these lay wait and lurk privily; certainly not their own, by any mode of construction.

And they lay wait for their [own] blood; they lurk privily for (o) their [own] lives.
(o) He shows that there is no reason to move these wicked to spoil the innocent, aside from their malice and cruelty.

And they lay wait for their own blood,.... While they lie in wait for the blood of others, they lie in wait for their own; and when they shed the blood of innocent persons, it in the issue comes upon their own heads, and is the cause of their own blood being shed; vengeance pursues them, and justice will not suffer them to live;
they lurk privily for their own lives: while they are lurking in secret places to take away, the lives of others, they are laying snares for their own souls; and the consequence of it will be, that they will be brought to a shameful and untimely end here, or, however, to everlasting ruin and destruction hereafter.

The causal conj. כּי (for) in Proverbs 1:16 and Proverbs 1:17 are coordinated; and there now follows, introduced by the conj. ו ("and"), a third reason for the warning:
And they lie in wait for their own blood,
They lay snares for their own lives.
The warning of Proverbs 1:16 is founded on the immorality of the conduct of the enticer; that of Proverbs 1:17 on the audaciousness of the seduction as such, and now on the self-destruction which the robber and murderer bring upon themselves: they wish to murder others, but, as the result shows, they only murder themselves. The expression is shaped after Proverbs 1:11, as if it were: They lay snares, as they themselves say, for the blood of others; but it is in reality for their own blood: they certainly lie in wait, as they say; but not, as they add, for the innocent, but for their own lives (Fl.). Instead of לדמם, there might be used לדמיהם, after Micah 7:2; but לנפשׁם would signify ipsis (post-biblical, לעצמם), while לנפשׁתם leaves unobliterated the idea of the life: animis ipsorum; for if the O.T. language seeks to express ipse in any other way than by the personal pronoun spoken emphatically, this is done by the addition of נפשׁ (Isaiah 53:11). המו was on this account necessary, because Proverbs 1:17 has another subject (cf. Psalm 63:10).

Their own blood - The destruction which they design to others, fall upon themselves.