Let their way be dark - Margin, as in Hebrew: "darkness." That is, let them not be able to see where they go; what danger they incur; what is before them. The idea is that of persons who wander in the night, not knowing what is before them, or what danger may be near. The succession of images and figures here is terrific. The representation is that of persons scattered as the chaff is before the wind; pursued by the angel seeking vengeance; and driven along a dark and slippery path, with no guide, and no knowledge as to the precipices which may be before them, or the enemies that may be pressing upon them.
And slippery - Margin, as in Hebrew: "slipperiness." This is a circumstance which adds increased terror to the image. It is not only a dark road, but a road made slippery by rains; a road where they are in danger every moment of sliding down a precipice where they will be destroyed.
And let the angel of the Lord persecute them - Pursue or follow them. The word "persecute" we use now in the sense of subjecting one to pain, torture, or privation, on account of his religious opinions. This is not the meaning of the word used here. It is simply to "follow" or "pursue." The image is that of the avenging angel following on, or pursuing them in this dark and slippery way; a flight in a dark and dangerous path, with a destroying angel close in the rear.

Let their way be dark - Let them lose their way, be entangled in morasses and thickets, and be confounded in all their attempts to injure me. All these phrases are military; and relate to ambushes, hidden snares, forced marches in order to surprise, and stratagems of different kinds.

Let their way be dark and slippery,.... In which they run before the angel, chasing and pursuing them; so that they know not where they are, at what they stumble, whither to flee, nor how to stand; the ways of wicked men are as darkness, they know not in what condition they are, and whither they are going; and utter darkness, even blackness of darkness, is reserved for them: but here it means a calamitous, uncomfortable, fickle, and unstable situation in this life; see Jeremiah 23:11. The allusion is to some of the valleys in the land of Palestine, which were dark, and the roads in them very smooth and slippery, as travellers in those parts have observed (q);
and let the angel of God persecute them; See Gill on Psalm 35:5.
(q) See Maundrell's Travel's, p. 7.

Their way - By which they flee, being chased.