You're a fucking idiot. Of course it was the punch that killed him, because it was the punch that set in motion a chain of events that led to him dying. What you're arguing is no different than saying that a guy who gets hit by a car doesn't die because the car hits him but because his head hits the pavement after the initial contact with the car. The guy is a victim of circumstances, but the circumstances were set in motion by the guy who hit him, plain and simple.
The guy may not have had intended to kill him, but his actions carried the implicit possibility that death could occur and it wasn't unreasonable at all to think that a sucker punch would lead to something like this. Just because the puncher was too stupid to realize this potential danger doesn't excuse him from anything other than a murder charge, hence the manslaughter charge instead.
Regarding the judge: he was spot on. This guy was a former amateur boxer and when a trained boxer makes the claim that he has a "killer punch" it clearly and overtly implies that he is aware of the potential his punches carry. So he wasn't at all unaware of the danger his punches carried. There was no negligence on his part as a result; he knew what his punches could do and he threw one. He may not have intended to kill the guy, but he intended to cause him serious harm and he knew the inherent danger in his actions. And of course the guy didn't have murderous intentions, otherwise he would have been charged with at least 2nd degree murder, and most likely 1st degree murder. An unprovoked, pre-meditated, intentional and successful attempt at murder would meet definitely meet the requirements defined by 1st degree murder. That's why he was convicted of manslaughter.
So in closing, you're a dumb motherfucker who should read about common law a little bit before you make outrageous, ignorant claims about the intelligence of a judge or the severity of an intentional, criminal action that resulted in the death of an innocent man. Get the fuck out.