1 Samuel 26

1 Samuel 26:10

David also said, "As the Lord lives, surely the Lord will strike him, or his day will come that he dies, or he will go down into battle and perish.

David is obviously content to take the long view in this situation, something that Saul is not able to do.  David has been told that he will be king, and Saul knows as well.  However, David is willing to wait until his time comes, while Saul is doing everything in his power to prevent the future he knows will happen from coming about.

Also, David continues to show reverence to the king by sparing him, but he does know that Saul's end will come.  He even thinks that God may well kill him directly, rather than letting natural causes or men do job.  Throughout all this, though, he will not be the instrument of Saul's demise; he knows that while something will happen, it won't be by his doing, even indirectly with an order to a subordinate.

1 Samuel 25

1 Samuel 25:43

David had also taken Ahinoam of Jezreel, and they both became his wives.

Is this David's first big mistake?

David already had a wife, the daughter of Saul.  Somehow, in the course of events (I don't remember if it came up before here), she had been given to another.  It's not clear (at least, not in my memory) if David divorced her, or if Saul simply took her away from David because of their conflict.  Regardless, at this point he has now taken an additional two wives.  Are David's hormones coming through?  Or does he think he's doing what any proper king should?  Either way, he's wrong, no matter what the local customs.