When the Philistine arose and came and drew near to meet David, David ran quickly toward the battle line to meet the Philistine. And David put his hand in his bag and took out a stone and slung it and struck the Philistine on his forehead. The stone sank into his forehead, and he fell on his face to the ground.
So David prevailed over the Philistine with a sling and with a stone, and struck the Philistine and killed him. There was no sword in the hand of David. Then David ran and stood over the Philistine and took his sword and drew it out of its sheath and killed him and cut off his head with it. When the Philistines saw that their champion was dead, they fled. And the men of Israel and Judah rose with a shout and pursued the Philistines as far as Gath and the gates of Ekron, so that the wounded Philistines fell on the way from Shaaraim as far as Gath and Ekron. And the people of Israel came back from chasing the Philistines, and they plundered their camp. And David took the head of the Philistine and brought it to Jerusalem, but he put his armor in his tent. (1 Samuel 17:48-54)
David has defeated Goliath. He invents the gun, on the spot, and essentially shoots his foe in the head, and Goliath falls on his face. The text tells us that David, using a sling and a stone, has killed Goliath. The next verse tells us that David ran and stood over the Philistine, took Goliath's own sword, drew it out of its sheath, and...killed him, cutting off his head. Did David kill him twice? No, the slingshot wound to the head was a killing blow, but not instantaneous. Even today, with more sophisticated weapons, gunshot wounds of low caliber to the head do not kill immediately, even though if left untreated the victim will die. This is what is happening here. David has killed Goliath with a small stone, but Goliath is still lingering. David finishes the Philistine off with Goliath's own sword, ending the life completely. This is why it says David killed him twice.
Christ defeated Satan through his death on the cross. This is the stone throw. The devil has lost. But God allows him time to deceive the nations until Christ comes over, picks up Satan's own sword and cuts the devil's head off with it. This "second killing" will be at Christ's second coming. We are living now in the meantime. We are living in the time where Jesus, without a sword of his own, is running over to finish the job. This seems like a lengthy jog over to the defeated devil to finish him off, but there is a reason for this. This age of our Lord, this intermediate time of the church, has a purpose. Look at Matthew 12:
And many followed him, and he healed them all and ordered them not to make him known. This was to fulfill what was spoken by the prophet Isaiah:
“Behold, my servant whom I have chosen,
my beloved with whom my soul is well pleased.
I will put my Spirit upon him,
and he will proclaim justice to the Gentiles.
He will not quarrel or cry aloud,
nor will anyone hear his voice in the streets;
a bruised reed he will not break,
and a smoldering wick he will not quench,
until he brings justice to victory;
and in his name the Gentiles will hope.” (Matthew 12:15-21)
A bruised reed, a smoldering wick: both refer to the soul of the believer, the one with faith in Christ. He is the "the poor" in the beatitudes. He is the one persecuted by the world, bruised, and his faith is barely that of a mustard seed, smoldering. The reason Christ delays the final killing blow is so that he may gather all of his flock together and not lose one, until his final victory is accomplished. As Peter writes in his second letter:
But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day. The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance. But the day of the Lord will come like a thief, and then the heavens will pass away with a roar, and the heavenly bodies will be burned up and dissolved, and the earth and the works that are done on it will be exposed. (2 Peter 3:8-10)
The Lord is patient with his drawing of the sword, so that his bruised reeds, his smoldering wicks might all come to repentance and be saved from the fire. Richard Sibbes writes in The Bruised Reed
The purpose of Christ's coming was to destroy the works of the devil, both for us and in us; and the purpose of the resurrection was, as well as sealing to us the assurance of his victory, so also (1) to quicken our souls from death in sin; (2) to free our souls from such snares and sorrows of spiritual death as accompany the guilt of sin; (3) to raise them up more comfortable, as the sun breaks forth more gloriously out of a thick cloud; (4) to raise us out of particular slips and failings stronger; (5) to raise us out of all troublesome and dark conditions of this life; and (6) at length to raise our bodies out of the dust. For the same power that the Spirit showed in raising Christ, our Head, from the sorrows of death and the lowest degree of his abasement, that power, obtained by the death of Christ from God, now appeased by that sacrifice, the Spirit will show in the church, which is his body, and in every particular member thereof. (p. 93)
Since therefore the children share in flesh and blood, he himself likewise partook of the same things, that through death he might destroy the one who has the power of death, that is, the devil, and deliver all those who through fear of death were subject to lifelong slavery. For surely it is not angels that he helps, but he helps the offspring of Abraham. Therefore he had to be made like his brothers in every respect, so that he might become a merciful and faithful high priest in the service of God, to make propitiation for the sins of the people. For because he himself has suffered when tempted, he is able to help those who are being tempted. (Hebrews 2:14-18)
David does not represent us but Christ among us, sharing our flesh, and being tempted as we are. The goal of Christ was to die on our behalf, because the one representing mankind needed to be fully human. Christ's death on the cross was the headshot, effectively killing the devil and delivering mankind from slavery to Satan. Many are still slaves, even to this day, because they have not put their faith in Christ. When we put our faith in our Lord and Savior, Jesus Christ, we will be spared at the end of the age. Those who follow Satan (and there are only the two paths) will flee and hide, like the Philistines, wishing the mountains to cover them. Christ delays the final "beheading" of Satan so that the nations can be fully deceived and his children can be fully converted. The line between the two sides needs to be sharp and defined. No one will say in the end, "I was dealt with unjustly." Christ brings Justice to Victory.