"But by the grace of God I am what I am" (1 Cor 15:10). Paul said this because he felt unworthy to be called an apostle. He persecuted God's people, yet God appeared to Paul and made him the last of the apostles. Paul, the murderer, was made an apostle not because he was qualified but because God chose to use him in this capacity.
David, centuries earlier, was humbled by God's grace just like Paul. 1 Samuel 25 reports that David had been good to Nabal's shepherds, giving them safe escort through the wilderness. In exchange for his services, David expected Nabal to repay good for the good he had given Nabal, especially during sheep sheering time when those who had much celebrated and gave generously to those who requested help. When Nabal unexpectedly (not unexpected from the standpoint of the reader - his name meant "fool"!) paid David for protecting his shepherds with scathing insult, David flew off the handle. "Every man strap on his swords!... God do so to the enemies of David and more also, if by morning I leave so much as one male of all who belong to him."
David was ready to slaughter. His response is not that different from ours when we do good for others only to get insult, unkindness and ingratitude in return. Our hearts are like David's heart: "Surely in vain have I guarded all that this fellow has in the wilderness, so that nothing was missed of all that belonged to him, and he has returned me evil for good."
Thank God the slaughter never took place. Abigail, Nabal's wife, met the angry band of sword wielding men head on and pleaded with them to respond to Nabal, the fool, with wisdom and discernment. But both David and Abigail knew better than to attribute the successful resolution of the conflict to smarts and good looks (the text does say Abigail is both beautiful and discerning). "Now then, my lord, as the LORD lives, and as your soul lives, because the LORD has restrained you from bloodguilt..." The Lord had graciously restrained from David from the sin of killing many innocent people. "But by the grace of God I am what I am."
Paul and David had the hearts of murderers and so do we. We are capable of the most wretched sins but God, according to his amazing grace, often restrains us. Praise God for all of the good in your life and for all of the disaster that has been averted. Plead with him to turn disaster, pain and sorrow into beauty. But by the grace of God we are what we are.