Neal was turning to drugs, he quit swimming, and became homeless when he was depressed because Lionel wouldn't forgive him.
This picture represents two friends making up like Lionel and Neal.
Matthew 6:14-15 For if you forgive other people when they sin against you, your heavenly Father will also forgive you. But if you do not forgive others their sins, your Father will not forgive your sins. If Lionel never took the time to talk to Neal to forgive his sins, then god would not forgive Lionel's sins.
Lionel was afraid to forgive Neal because he thought that he would be letting him off the hook for killing his parents.
Man Down talks about how she feels bad about killing a guy. She can't sleep at night knowing she took a man's life. Which is the same way that Neal feels.