David swore to God – and he was serious, passionate, dedicated in seeing it through. I wonder, have I made similar promises (most probably), and have I kept those promises with the same fervor (most probably not).
God swore to David as well – and for sure He kept it. Regardless of David’s action, God will keep (has kept) His promise. Not because of our sake, but because of His grace. Because He wants to bless – it is His desire. No one can tell Him otherwise.