How is Dr. Jekyll secret revealed at the end? Do you think his end is justified?

The first part of your question has already been answered. Dr. Jekyll's complete transition to Hyde was in my opinion justified. He toyed with evil and enjoyed it. He continued while knowing the repercussions, and even though he sporadically attempted to stop, he never held to that decision because he enjoyed being Hyde and was bored with his own life. Jekyll only panics when he finds he cannot tranform at will..... start it and end it as he pleases. He's happy as long as he is in control, but the truth is.... he's never really been in control.