that the hand of the Philistines may be against him

Harming someone is spoken of as if it were placing one's hand on that person. Here, Saul is referring to killing David. See how you translated a similar phrase in 1 Samuel 18:17. AT: "so that the Philistines may kill him" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-euphemism)

You will be my son-in-law

"You will be the husband of my daughter"