Saul's outburst reveals a deep insecurity. He's not just angry; he's terrified of losing everything – his throne, his legacy, and his reputation.
Saul’s rage is magnified by his own paranoia and inflated sense of self. He believes David is a threat, not just to his life, but to his entire royal standing. The phrase 'do I not know that you have chosen the son of Jesse' shows Saul’s conviction that he understands Jonathan’s motivations, yet he twists those motives into something shameful. His pride cannot bear the thought that his son would align with someone he perceives as beneath him (Jesse's son) and elevate that person over the king. This is the pride of a man whose identity is deeply wrapped up in his position and power, and who fears losing it all.