The way they play the game
By now you have seen the video. Probably have watched it several times - in real time and in slow motion. Zdeno Chara rides Canadiens winger Max Pacioretty into a stanchion and Pacioretty drops to the ice. My first thought at the sight of the video before I knew the result, "Is he dead?" Chara doesn't strike many people as the type of guy who would intentionally injure someone. Intentionally ferociously check, yes. Injure, no.
Did he deserve a suspension? Let's be frank - Pacioretty will not play hockey for a long time, perhaps never again. Hockey is a tough, dangerous sport; no league can legislate complete safety. Without evidence as to intent, it's hard for me to insist on punishment despite the dramatic injury.