Jamieson, Fausset and Brown on 1 Samuel 21:2

2. The king hath commanded me a business, and hath said unto me, Let no man know--This was a direct falsehood, extorted through fear. David probably supposed, like many other persons, that a lie is quite excusable which is told for the sole purpose of saving the speaker's life. But what is essentially sinful, can never, from circumstances, change its immoral character; and David had to repent of this vice of lying (Psa 119:29).