From the sound of his comments, it appears as though those making the threats were not particularly credible, and that they mistakenly believed the fan page to be an official site run by Warner Bros. It's unclear exactly who was threatened, although it doesn't appear to have been Affleck himself.
The page, run by fans and not connected to the official Man of Steel Facebook page, has been a place where fans have been coming for months to talk (and often disagree) about Man of Steel and other Warner Bros./DC movies and comics. As an unabashed fan site that has tried to stay positive throughout the process, the site has found itself targeted in the past by fans unhappy with Man of Steel, but apparently never to the extent that this week's casting notice has driven things.
"Its one thing to express a casting isn't your favorite choice, to be respectfully critical. It’s quite another, and totally unwarranted, to message death threats to the page admins over it or to post extreme vulgarity concerning it," wrote Leslie [last name not given], who runs the site.
The page added that links to petitions to fire Affleck would be taken down from their Facebook wall, as well as posts containing too much vulgarity or "excessive hyperbole." They asked fans to find more constructive ways of conveying their displeasure.
"One threatening message cited a petition in his threat as a reason for his threat to be carried out," Leslie wrote. "While I don’t believe the threats were credible, they also are absolutely not going to be accepted here. This isn't the typical type of trolling/ flaming we see. This is over-the-top and it has to stop on this page."