Charles Ellicott Commentary 1 Samuel 20:31

"For as long as the son of Jesse liveth upon the ground, thou shalt not be established, nor thy kingdom. Wherefore now send and fetch him unto me, for he shall surely die." — 1 Samuel 20:31 (ASV)

You shall not be established. — Here the king gives expression to the thought that was always torturing that poor diseased brain of his—David, his own kind physician, his faithful soldier, and his son’s dearest friend and loved companion, was plotting despicably against that master for whom he had done so much, and the son whom he loved so well.

Saul, in his blind fury, goes on to betray his deadly purpose when he exclaims, “he shall surely die.” His command, “Send and fetch him to me,” tells us that the murder had been pre-arranged to take place at the feast. Doubtless those rough soldier chiefs sitting around the royal table would be ready at any moment to carry into effect their master’s savage command.