A divided U.S. Supreme Court on Thursday blocked the execution of a killer whose lawyers argued that he should not be put to death because he is mentally ill. The court ruled 5-4 in the case of Scott Louis Panetti, who shot his in-laws to death 15 years ago in front of his wife and young daughter. The convicted murderer says that he suffers from a severe documented illness that is the source of gross delusions. "This argument, we hold, should have been considered," said Justice Anthony Kennedy, who wrote the majority opinion. Panetti's lawyers wanted the court to determine that people who cannot understand the connection between their crime and punishment because of mental illness may not be executed.