Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
And it came to pass, when the Philistine arose, and came and drew nigh to meet David, that David hasted, and ran toward the army to meet the Philistine.
And David put his hand in his bag, and took thence a stone, and slang it, and smote the Philistine in his forehead, that the stone sunk into his forehead; and he fell upon his face to the earth. 
So David prevailed over the Philistine with a sling and with a stone, and smote the Philistine, and slew him; but there was no sword in the hand of David. Therefore David ran, and stood upon the Philistine, and took his sword, and drew it out of the sheath thereof, and slew him, and cut off his head therewith.
And when the Philistines saw their champion was dead, they fled. And the men of Israel and of Judah arose, and shouted, and pursued the Philistines, until thou come to the valley, and to the gates of Ekron. And the wounded of the Philistines fell down by the way to Shaaraim, even unto Gath, and unto Ekron. And the children of Israel returned from chasing after the Philistines, and they spoiled their tents.
And David took the head of the Philistine, and brought it to Jerusalem; but he put his armor in his tent. And when Saul saw David go forth against the Philistine, he said unto Abner, the captain of the host, Abner, whose son is this youth?
And Abner said, As thy soul liveth, O king, I cannot tell. And the king said, Enquire thou whose son the stripling is. And as David returned from the slaughter of the Philistine, Abner took him, and brought him before Saul with the head of the Philistine in his hand. And Saul said to him, Whose son art thou, thou young man? And David answered, I am the son of thy servant Jesse the Bethlehemite.
Vs.48-58
Here is
1. The engagement between the two champions:
He hasted, and ran, being lightly clad, to meet the Philistine.... throwing himself into the mouth of
destruction, but he knew whom he had believed and for whom he acted.
2.
The fall of Goliath in this engagement:
One of the Rabbin thinks that when Goliath said to David, Come, and I will give thy flesh to the fowls of the air, he threw up his head so hastily that his helmet fell off, and so left his broad forehead a fair mark for David.
Smote the Philistine in his forehead — Except his face, Goliath was everywhere covered over with strong armor. Either he had no beaver
to his helmet, or it was lifted up so as to expose his forehead; but it
does not appear that the ancient helmets had any covering for the face.
Though he had been at court formerly, yet, having been for some time absent,
Saul had forgotten him, being melancholy and mindless, and little
thinking that his musician would have spirit enough to be his champion;
and therefore, as if he had never seen him before, he asked whose son he
was. Abner was a stranger to him, but brought him to Saul, and he gave a modest account of himself.
And now he was introduced to the court with much greater advantages
than before, in which he owned God's hand performing all things for him.
Behold the giant's head taken off with his own sword.
Let it be before your eyes for a sign.
