Questions about Zajac from a Newcomer to Boss

I am up to episode 4 of season 1, and I have a question about Zajac that is troubling me. Governor Cullen releases a potentially damning video of Zajac 20 years ago involved in an attack of a black man. Zajac's explanation is that he ran to get help, but when he returned the black man was beaten nearly unconscious. I couldn't make out the film footage, but is that Zajac in the video beating the man while he is on the ground? Are we expected to believe that Zajac's actions in the video are so ambiguous that he was able to spin the story to make it seem as if he was trying to help the man? Why is he calling the black man his "friend"? If the story is untrue, how does Zajac get the black man to go along with the story? I rewound scenes with Zajac several times and I can't find where these questions are explained. Did I miss some key plot device? I am only up to episode 4, so perhaps the answers to my questions come later in the season. I would much appreciate it if anyone could shed light on these questions for me. Don't worry about spoilers.