The moral of slingblade is clearly sacrificing your own happiness and shunning your own needs for the benefit of others. It seems to me that the events of the movie were Karl's way of seeking out redemption for being unable to save his little brother. Although there was nothing he could have done for the premature infant, I can imagine the guilt that must have torn him up inside every day. So I believe that Karl felt a moral obligation to help Frank not only out of care for the boy, but also wanting to clear his conscience.

Karl can be seen as a sacrificial figure because even after he had just regained his freedom, he was willing to sign himself up for lifelong incarceration for the benefit of a child in need. Some may argue that Karl didn't have much to live for, but I would like to point out that he had an excellent job for someone with his mental disabilities and his relationship with Melinda seemed promising. So in truth, Karl gave up just as much as you or I would have had to.