You can't get to where God wants you to be until you become who He wants you to be. His primary concern is not whether you accomplish a particular goal, rather He is concerned with who you are becoming on the way. Your personal integrity is a key element in this. David has the opportunity to kill Saul when he went into the cave alone where David and his men were hiding. From the world's point of view, it was a God given opportunity to kill the man who was trying to kill him and become the king that God had promised he would be. But David's integrity would not allow him to do something wrong to accomplish what was right. An opportunity isn't an opportunity if you have to compromise your integrity. David passed God's test of his integrity and showed he was becoming the man God wanted him to be. This was a critical element in God's preparing David to be the great king He wanted him to be.