Is it libelous?

1. “Megan Fox is a man!” Headline on Weekly World News Web site
No, because this is a hyperbole.

2. “Up until the day he died, he was a brilliant writer. But the drugs made him a thief, a pimp and a liar,” said friend Karen Smith, who was with Johnson at the time of his death.
Not libelous, because it is a quote and the opinion of Karen Smith. We can also say whatever we want if the subject is dead.

3. “In my opinion, Kevin is a murdering rapist,” the prosecutor told the jury.
No, because the man is speaking to the jury and journalists can cover what is being said in a courtroom..

4. "In my opinion, he's a murdering rapist," the man said at the rally.
Yes