The fans (or at least many of them) WANT the players to beat the living hell out of each other, that's one of the primary draws of the sport in the first place. You constantly hear players and commentators bemoaning all the rules that have "softened" the game too much, especially around how daintily the QB's are treated and how all the rules are geared in their favor.
Players are supposed to "fly to the ball", when you have 3 defensive players all converging on one ball carrier at full speed, how can you consistently keep from hitting someone in the head? It's not possible. Huge men running at full speed and colliding with each other is inherently dangerous and that's just the way it is.
Don't get me wrong, I personally hate to see players get injured, but it's part of the game and always has been. It's a violent fucking sport, the players know this. The fans know this...and that's why many of them watch it. People love to see "the big hit", they love to see someone get taken down and then have to be helped off the field. Sure, they probably wouldn't admit to that, but it's the truth.