Jamieson, Fausset and Brown on Job 7:20

20. I have sinned--Yet what sin can I do against ("to," Job 35:6) thee (of such a nature that thou shouldst jealously watch and deprive me of all strength, as if thou didst fear me)? Yet thou art one who hast men ever in view, ever watchest them--O thou Watcher (Job 7:12; Da 9:14) of men. Job had borne with patience his trials, as sent by God (Job 1:21; 2:10); only his reason cannot reconcile the ceaseless continuance of his mental and bodily pains with his ideas of the divine nature.

set me as a mark--Wherefore dost thou make me thy point of attack? that is, ever assail me with new pains? [Umbreit] (La 3:12).