Tackling someone at head level is illegal for one reason that goes well beyond safety. It's not football. Football is about the confrontation of athletes, at the top of their abilities, and may the best men win. When you get hit in the head, all of your determination and concentration on advancing that ball and winning that play, are instantly sapped out of your body. In a flash, every ounze of your energy transforms from trying to win a football game to preserving the safety of your head and neck. I played two years of organized football when I was a teenager. I was small, not a great player, but I knew how to play the game. I can attest, it's a hell of a scary moment when somebody takes your head out. You can be the better of the two men on the field confronting each other but if your opponent tackles or hits your head, you are going down, period, end of conversation.
For a closer look at a famous head shot, watch this video of the most infamous and saddest head hit in NFL history. One hit to the head of Daryl Stingley deprived him for life of the use of his limbs and contributed to his premature death at age 55.