Michal, Saul's daughter, loved David: and they told Saul, and the thing pleased him.
"The king desires no marriage present except a hundred foreskins of the Philistines"  (1 Samuel 18:25).  The Philistines were an uncircumcised people; and "the foreskin" was the part of the body cut off in the ceremony of circumcision.  The delivery of a hundred such things would have been possible only by first killing a hundred Philistines.  Saul, of course, knew this; and this was precisely the part of the arrangement which Saul relied upon to accomplish the murder of David by the hand of the Philistines.