Whoso diggeth a pit shall fall therein. This thought is found often elsewhere; e.g. Psalms 7:16; Psalms 9:16; Ecclesiastes 10:8; Ecclesiasticus 27:25, 26. The pit is such a one as was made to catch wild animals; the maker is supposed to approach incautiously one of these traps, and to tall into it. And he that rolleth a stone, it will return upon him. This does not refer to throwing stones into the air, which fall upon the head of the thrower, but to rolling stones up a height in order to hurl them down upon the enemy (comp. 9:53; 2 Samuel 11:21). Of such retributive justice we have numerous examples;e.g. Haman hung on the gallows which he had prepared for Mordecai (Esther 7:9, etc.). So the old story tells how Perillus, the inventor of the brazen bull in which prisoners were to be burned alive, was himself made to prove the efficacy of his own invention by the tyrant Phalaris; as Ovid says
"Et Phalaris tauro violenti membra Perilli
Torruit; infelix imbuit auctor opus."
('Art. Amat.,' 1.653.)
So we have, "Damnosus aliis, damnosus est sibi;" ἡ δὲ βουλὴ τῷ βουλεύσαντι κακίστη. St. Chrysostom speaks of the blindness of malice: "Let us not plot against others, lest we injure ourselves. When we supplant the reputation of others, let us consider that we injure ourselves, it is against ourselves that we plot. For perchance with men we do him harm, if we have power, but ourselves in the sight of God, by provoking him against us. Let us not, then, injure ourselves. For as we injure ourselves when we injure our neighbours, so by benefiting them we benefit ourselves" ('Hom. 14, in Phil.,' Oxford transl.).