I watched clips of the movie in high school English class, and thought that none of the extremely farfetched doubts that Fonda cast seemed to be reasonable. By the end of the class I thought that Fonda was just a manipulative scumbag. My English teacher got mad at me for saying so, and gave me a C- on my essay.
I had only seen bits and pieces of the film before watching the whole thing in one sitting about a month ago. Initially as a teenager, my only takeaway was that Fonda was a huge liberal douche and I still thought the kid was guilty. But once I was done watching the entire thing from start to finish, I'm actually convinced that Fonda's character was a psychopathic genius. He was the one who murdered that guy, and he was there to taunt the legal system.
Seriously, go and watch the movie again, but this time go into it with the knowledge that Fonda's character is a Hannibal Lecter-level, high IQ killer who actually murdered that guy just to see what would happen, and now he's on the jury trying to fuck with people for fun. The movie, and his character's motivations start making a lot more sense when you see him as a sick bastard who gets his rocks off by going to bad neighborhoods and killing random minorities, then acting sanctimonious and defending the very people he's framed.