If you read
1 Samuel you'll discover that
Saul is continually jealous and suspicious of
David. I always interpreted this passage as an example of an "impossible task" motif of
folklore: the
father who doesn't like a particular young man gives him an apparently impossible task--usually one that is expected to result in his death. Of course, the young man goes out and achieves this impossible task--and in this case, David goes the extra mile and kills another hundred
Philistines, just to make his point clear.