Charles H. Spurgeon on Psalms 69:27

Add iniquity unto their iniquity. Unbelievers will add sin to sin, and so, punishment to punishment. This is the severest imprecation, or prophecy, of all. For men to be let alone to fill up the measure of their iniquity, is most equitable, but yet most awful. And let them not come into thy righteousness. If they refuse it, and resist thy gospel, let them shut themselves out of it.

"He that will not when he may,

When he would he shall have nay."

Those who choose evil shall have their choice. Men who hate divine mercy shall not have it forced upon them, but (unless sovereign grace interpose) shall be left to themselves to aggravate their guilt, and ensure their doom.