John Wilkes Booth and the Question of Motivation

How do we really know for sure what motivated John Wilkes Booth? Barry Rubin raises some troubling questions:

John Wilkes Booth opened fire on President Abraham Lincoln in Ford's Theatre in April 1865, the media was puzzled. "True, the actor was outspoken in his Confederate sympathies and viewed himself as a Southerner," said someone who knew him, "but that was no reason he might want Lincoln to be dead." The day before he went on his shooting spree, Booth hoisted a big Confederate flag outside his hotel room. After he leaped onto the stage he shouted, "Thus ever to tyrants!" the motto of the rebel state of Virginia.