Why did the young man lie about killing saul?

The young man lied about killing Saul to gain favor and recognition, believing that his claim would impress David, Saul's successor. He likely thought that presenting himself as a hero who ended Saul's reign would secure a reward or position of power. However, this act of deceit ultimately led to his own demise, as David was deeply saddened by Saul's death and punished the young man for his falsehood.