Ernie Takes it to the Next Step

There's a novel where a serial killer is hanging out in a park one day. A basketball player gets hit in the face and has a nose bleed. The killer offers his handkerchief to sop up the blood. Later the killer buries a body and leaves the handkerchief the corpse. The totally innocent basketball player is arrested. The killer then impersonates a defense attorney and takes the case. Naturally he loses. He visits the convicted man up until the day of the execution.