John Calvin Commentary Psalms 58:8

"[Let them be] as a snail which melteth and passeth away, [Like] the untimely birth of a woman, that hath not seen the sun." — Psalms 58:8 (ASV)

Let him vanish like a snail, which melts away: the two comparisons in this verse are introduced with the same purpose as this one, expressing his desire that his enemies might pass away quietly and prove to be things in their own nature most evanescent. He compares them to snails, and it might appear ridiculous for David to use such contemptible comparisons when speaking of men who were formidable for their strength and influence, if we did not reflect that he considered God as able, in a moment, without the slightest effort, to crush and annihilate the mightiest opposition.

Their power might be such as encouraged them, in their false confidence, to extend their schemes into a far distant future; but he looked upon it with the eye of faith and saw it doomed in the judgment of God to be of short duration. He perhaps alluded to the suddenness with which the wicked rise to power and intended to shatter the pride they are likely to feel from such an easy advance to prosperity, by reminding them that their destruction would be equally rapid and sudden.

There is the same force in the comparison used at the end of the verse, where they are compared to an abortion. If we consider the length of time to which they, in their false confidence, expect their lives to extend, they may be said to pass out of this world before they have truly begun to live, and to be dragged back, so to speak, from the very goal of existence.