He’ll Start Off As The Antagonist

At its heart, Batman v Superman: Dawn of Justice is essentially a sequel to Man of Steel. That’s what it’s being marketed as and Henry Cavill is the top-billed name on all marketing materials. With that in mind, there’s every reason to assume that his Superman will be the movie’s protagonist – at least to begin with.
That means that this Batman will be in the unusual position of being the antagonist from the moment the credits roll at the beginning.
He’ll be going after Superman, believing him to be a threat to the world – but audiences will know otherwise. Clark Kent is a decent guy with good morals and intentions – publicity in the DC Cinematic Universe and the government being wary of him will lead to him getting a bad image in the press. The audience will, therefore, see Batman as the villain initially.
It will be somewhat strange to see people rooting against the Dark Knight, but that will change fairly quickly when he eventually joins forces with Superman and forms the Justice League.
On that note…