Adam Clarke on Psalms 55:15

Let death seize upon them - This is a prediction of the sudden destruction which should fall on the ringleaders in this rebellion. And it was so. Ahithophel, seeing his counsel rejected, hanged himself. Absalom was defeated; and, fleeing away, he was suspended by the hair in a tree, under which his mule had passed; and being found thus by Joab, he was despatched with three darts; and the people who espoused his interests were almost all cut off. They fell by the sword, or perished in the woods. See 2Sam 18:8.

Let then go down quick into hell - Let them go down alive into the pit. Let the earth swallow them up! And something of this kind actually took place. Absalom and his army were defeated; twenty thousand of the rebels were slain on the field; and the wood devoured more people that day than the sword devoured, 2Sam 18:7, 2Sam 18:8. The words might be rendered, "Death shall exact upon them; they shall descend alive into sheol." And death did exact his debt upon them, as we have seen above.