The word "hand" is a metonym for the person's power. "Yahweh gave me an opportunity to attack you today" or "Yahweh placed me where I could easily have killed you" (See: [[rc://en/ta/man/translate/figs-metonymy]])
David speaks as if Saul were another person to show that he respects Saul because Saul is king. AT: "the one he has chosen to be king" (See: [[rc://en/ta/man/translate/figs-pronouns]])