Let destruction come upon him at unawares - Margin, which “he knoweth not of.” So the Hebrew. The meaning is, Let destruction come upon him when he is not looking for it, or expecting it.
And let his net that he hath hid catch himself - See the notes at Psalms 7:15-16. The psalmist prays here that the same thing may occur to his enemy which his enemy had designed for him. It is simply a prayer that they might be treated as they purposed to treat him.