Jamieson, Fausset and Brown on 2 Samuel 12:5

5. the man that hath done this thing shall surely die--This punishment was more severe than the case deserved, or than was warranted by the divine statute (Ex 22:1). The sympathies of the king had been deeply enlisted, his indignation aroused, but his conscience was still asleep; and at the time when he was most fatally indulgent to his own sins, he was most ready to condemn the delinquencies and errors of others.

2Sa 12:7-23. He Applies It to David, Who Confesses His Sin, and Is Pardoned.