According to the book that I read (and described above
) the Kennedys knew that Marilyn took prescription drugs on a regular basis and vey often with alcohol to help her sleep and so when the Mob let themselves into her house to kill her she was already out for the count and wouldn't have heard them or offered any resistance. Then they injected the lethal dose into the skin in her perinium as a) no one would have looked there for a needle mark b) it was highly unlikely that a needle mark would have remained given the type of skin in this area c) a lot of people knew that she was unhappy and was taking a lot of prescription pills and would have assumed that suicide was the cause of death.
