From Hyman Hayden:
The Martha Moxley case is unusual because the man on trial, Michael Skakel, is related to the Kennedy family, and because the crime took place more than 25 years ago, when both Skakel and Moxley were 15 years old. Subsequently, Skakel became an alcoholic, and entered an abuse treatment center where, according to Coleman, Skakel confessed to killing Martha in a jealous rage when he saw her making out with his brother. This proves Skakel is an idiot. Why not just kill his brother? That way, he'd have a better chance with the girl.