Charles Spurgeon 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)

Saul knew that David, and not Jonathan, was to succeed him on the throne.

He gives Jonathan warning of that fact, yet seeks his rival's life.