Sonny isn't the only dumb one. Commissioner Raines wasn't exactly clicking on all cylinders when he decided to charge Sonny with the Deimos murder. Currently, the only evidence against the Big Boy is his confession. Nobody saw him do it, his prints and DNA aren't on the knife, and there isn't a spot of blood on his clothing. In short, all the Salem P.D. has is an uncorroborated confession, which isn't good enough. In real life, If the police arrested all the people who confess to major crimes, the jails would be jam-packed.