MM was murdered and the engine behind that was Ethel Kennedy (a notoriously evil dictatorial bitch) and the various government entities who served the Kennedy family. MM was out of control re her obsession with Bobby and called the WH aso many times she becamse a stalker. She refused to stop and refused to accept the affair was over, she was truly devastated. She did make threats to go public with very sensitive critical information she had gleaned from Bobby and Jack and was thus cconsidered a threat against national security.
Peter Lawford knew what MM knew, but of course, was far more circumspect and waqs not emotionally involved with anyone. He knew nothing about murder versus suicide, but he did receive a phone call ordering him to go to MM's house and "clean it" meaning, remove and and all info which could implicate or embarrass the Kennedys. So that's what he did. He went to his grave without ever revealing what he knew and there is an excellent book about him called "The Man Who Kept the Secrets." well worth reading.