For an explanation the first part of this chapter, I quote from the note of The Lutheran Study Bible with the emphasis added. “Saul’s sons, whether innocent or guilty with their father, now pay the penalty for Saul’s crime. God will not allow sin to go unpunished, though His justice sometimes does not make sense to us. The apparent innocence of the seven from Saul’s house, who were hanged on a mount before the Lord as an act of atonement for Saul’s sin, brings into sharp focus the all-atoning sacrifice of Christ, the Son of God, who was crucified on Mount Calvary for all the sins of the world. Not just any man’s death would propitiate (win the favor of) God; only the death of God’s Son could bring salvation.”
During the second half of the reading, we see that David has become a target for the might men of the Philistines, and David’s men would no longer allow him on the field of battle. Instead, they fought the great men of the Philistines (who were descendents of the giants) in his place defeating them again. God continued to guide David and his men.