Adam Clarke on 1 Samuel 27:2

David arose, and he passed over - unto Achish - There is not one circumstance in this transaction that is not blameable. David joins the enemies of his God and of his country, acts a most inhuman part against the Geshurites and Amalekites, without even the pretense of a Divine authority; tells a most deliberate falsehood to Achish, his protector, relative to the people against whom he had perpetrated this cruel act; giving him to understand that he had been destroying the Israelites, his enemies. I undertake no defense of this conduct of David; it is all bad, all defenceless; God vindicates him not. The inspired penman tells what he did, but passes no eulogium upon his conduct; and it is false to say that, because these things are recorded, therefore they are approved. In all these transactions David was in no sense a man after God's own heart. Chandler attempts to vindicate all this conduct: those who can receive his saying, let them receive it.