Hoist on his own Petard

When it was decided that David would take on the mighty Goliath, the king offered to gird David with his royal sword and armor, but these would not be David's weapons.

Saul clothed David in his own garment; he placed a bronze helmet on his head and fastened a breastplate on him. David girded his sword over his garment. Then he tried to walk in these but was not used to it...So David took them off. He took his stick, picked a few smooth stones from the wadi, put them in the pocket of his shepherd's bag and sling in hand, he went toward the Philistine. (1 Samuel 17:38-40)

Goliath's power ideology did not scare David:

You come against me with sword and spear and javelin; but I come against you in the name of the Lord of Hosts the God of the ranks of Israel, whom you have defied...For the battle is the Lord's and He will deliver you into our hands. (1 Samuel 17:45)

After Goliath had been felled with a single stone from his slingshot, David ran over to the Philistine, pulled Goliath's sword from its scabbard, dispatched him, and cut off his head. The just desserts of the uncircumcised Philistine find expression in the words of the Psalmist:

The wicked draw their swords, bend their bows, to bring down the lowly and needy, to slaughter upright men. Their swords shall pierce their own hearts and their bows shall be broken. (Psalms 37:14-15)