I've read Leonard's autobiography too and agree it's a good read.
Don't get me wrong - in the boxing terms the decision was not controversial. It was a very close fight. The punch count was very close also. It favoured Leonard because Hagler virtually gave away the first 3 rounds fighting orthodox (he was a natural southpaw). For the remainder of the fight, Hagler was more than holding his own and was the heavier punchier. Leonard was hurt several times.
Leonard also admitted part of his plan was to really steal rounds by influencing the judges by being very active in the last 30 seconds of each round.
If you watch the fight with the commentary off and forget who the fighters are then I can't see how the fight is scored in Leonard's favour.
Turn the commentary back on and also note that Leonard is performing way above the pre-fight expectations and Hagler some way below. That's very influential to judges (even though it shouldn't be)... plus the crowd was firmly pro-Leonard.
Nobody would deny that Leonard fought a great fight in context that he hadn't fought for a while (although apparently he had fights behind closed doors against top pros as part of his build up - far beyond usual sparring sessions). Nobody would deny he fought way above expectations. Nobody will forget it was his first fight at the weight. His preparation, tactics and strategy were spot on.... He was superb. BUT IMO, he didn't win the fight. Again, it was not controversial (other than one idiot judge who gave it to him by 8 rounds - now that WAS criminal) - Overall, I've seen far worse decisions, including many where a fighter has been plainly robbed. This decision wasn't a robbery at all.... it's just I scored it for Hagler.
I think there was a good chance Hagler would have retired even if he had beaten Leonard. He was past his best and had considered retiring after the Mugabe fight. It was watching the Mugabe fight that Leonard finally believed he could take him. He'd avoided him like the plague when Marvin was in his prime in the early 80s.