Her death was highly suspicious
Out of all the celebrity death conspiracy theories, Marilyn Monroe's death is one of the most suspicious and most often debated. (Second only, perhaps, to Elvis Presley.)
Monroe died on August 5, 1962 at the age of 36. Her death was ruled "acute barbiturate poisoning" by the LA County Coroner. Everyone agrees that Monroe died from an overdose of barbiturates, but there are enough inconsistencies in her death to raise suspicion. Several LAPD detectives were convinced at the time she was murdered, and the suspicious circumstances of her death remain a perennial topic today.
First, the conventional explanation. Monroe had developed a serious dependency on Nembutol, a barbiturate which is so lethal that it is used today to euthanize animals and as one of several drugs used to execute prisoners in the United States.
Monroe's dependency had grown so bad that her doctor was trying to wean her off the stuff and onto chloral hydrate (which is so dangerous that it is has not been approved for use by the FDA or in Europe). Ostensibly she took the drugs for insomnia, but in reality it had gotten to the point where she was taking them around the clock.
It would be pretty easy for Monroe to have died due to an overdose, either accidental or intentional. However, there are a lot of inconsistencies in her death, and in the report given by the housekeeper Eunice Murray who was there at the time, and who cleaned up Monroe's room (including changing the linens) after finding her dead but before phoning the police to report the death.
Police noted that although they found empty pill bottles, there was no water glass at the scene of the crime. A water glass later appeared on the floor, in an area which the police claimed to have searched. Monroe had a well-known difficulty swallowing pills, and would often gag on them even if she had water. It's highly unlikely she would have dry-swallowed two entire bottles of pills.
Her internal organs did not show signs of Nembutal poisoning, which you would expect if she had swallowed the pills. This has led some people to believe that the drugs had been injected into her bloodstream. Although the county coroner was ordered to investigate, no record of his findings was kept. The toxicologist destroyed Monroe's organs after his initial report.
But who stood to gain from Monroe's death? One theory is that she had threatened to go public about her affair with John F. Kennedy, and that she was presumably killed by the Secret Service to keep her quiet. Another theory is that she was killed by the Illuminati because, aging and increasingly insane, she had outlived her usefulness to the Hollywood machine.