I don't really understand how all this works so bear with me.
Hernandez had severe brain damage. That damage arises from head injury. That head injury happens in football.
None of this is disputed, and I think it's crystal clear that his CTE came from football, not from some other activity. So him playing football was the cause of his brain damage. Let's even go so far as to say that this brain damage caused by playing football was what led him to a violent life.
Now, even if all that was true, how could Hernandez' estate win a suit against the NFL or Riddell? First, with respect to Riddell, Riddell is trying to make the game safer; they're not trying to trick people into thinking the game is safe while it not *really* being safe. They're continually developing new, safer helmets. How is the guy who is working his ass off to make the game safer responsible for the game being unsafe?
Second, the only way the NFL could be complicit (guilty, responsible, whatever word we want to use) in this is if they knew the game was unsafe, but told the players it WAS safe, right? If the NFL didn't know it wasn't safe, and the government gave the NFL approval to exist (which it does, even giving it the antitrust exemption), then how could they be responsible? If the NFL knew that it's a dangerous sport that could lead to brain injuries and Hernandez *also* knew it was a dangerous sport that could lead to brain injuries, but played the game willingly and voluntarily anyway (nobody forced him to play football), then how could the NFL be responsible? It's only if the NFL knew the game was dangerous for the head, and kept that information from players, or if it somehow made the players play, that the NFL could be responsible. That's how I see it. Kind of what like got tobacco companies in trouble. It wasn't that smoking was a contributing cause of lung cancer; it's that they KNEW it did, but hid that information, so they were basically deceiving their customers.
Now, unfortunately for the NFL, in some ways, this is exactly the case. We have mounting evidence that the NFL understood that it has a massive problem with CTE and even manipulated research and hid data that would reveal the extent of the problem. I don't know the timeline however. Did all this take place before or after Hernandez played in the NFL? It's all fuzzy to me.
But that's the only way I could see Hernandez' estate winning, even though your point, crystalline, is spot on.