Jamieson, Fausset and Brown on Ecclesiastes 7:16

16. Holden makes Ec 7:16 the scoffing inference of the objector, and Ec 7:17 the answer of Solomon, now repentant. So (1Co 15:32) the skeptic's objection; (1Co 15:33) the answer. However, "Be not righteous over much," may be taken as Solomon's words, forbidding a self-made righteousness of outward performances, which would wrest salvation from God, instead of receiving it as the gift of His grace. It is a fanatical, pharisaical righteousness, separated from God; for the "fear of God" is in antithesis to it (Ec 7:18; 5:3, 7; Mt 6:1-7; 9:14; 23:23, 24; Ro 10:3; 1Ti 4:3).

over wise--(Job 11:12; Ro 12:3, 16), presumptuously self-sufficient, as if acquainted with the whole of divine truth.

destroy thyself--expose thyself to needless persecution, austerities and the wrath of God; hence to an untimely death. "Destroy thyself" answers to "perisheth" (Ec 7:15); "righteous over much," to "a just man." Therefore in Ec 7:15 it is self-justiciary, not a truly righteous man, that is meant.