John Wesley on 1 Samuel 20:16

A covenant - The covenant which before was personal, he now extends to the whole house of David, expecting a reciprocal enlargement of it on David's side, which doubtless he obtained. Enemies - If either I or any of my house shall break this covenant, and shall prove enemies to David or to his house, let the Lord, the witness of this covenant, severely punish the violators of it.