You could not possibly pay off enough persons to keep this quiet and once they start talking about criminal negligence (IE: prison), people will squawk very loudly.I have to agree with TopT, too many to keep it a secret. Sloppy, criminally negligent? Yes. An effective cover up? No. The civil cases against his estate and possible criminal cases against his enablers will go on for so long everyone will become bored.
To keep everything under wraps you would have to murder literally hundreds of people, victims, enablers and prison staff.