This is a tough one. In my opinion Ovechkin triggered the whole thing off with his blatant careless use of his stick with that awful slash. That was definitely a penalty. This action led to Crosby going off balance & directly into Niskanen's path and who was setting up to hit Crosby. Niskanen likely wasn't targeting the head when he made the decision to hit Crosby (which was his job, by the way) but the way Crosby involuntarily turned into him caused the head to be struck. I don't believe Niskanen had enough time to react. It was the type of play which took a split-second to happen in the heat of playoff battle.
Ovechkin should have got a penalty for slashing. I'm okay with Niskanen getting a roughing penalty. But although i'm slightly siding with the Penguins in this series (although i put money on Washington to make it to the finals), i don't believe Niskanen deserves to be suspended. It was an unfortunate play and there is a lot of hoopla about it because it's Sidney Crosby, it's the playoffs and he has a long history of concussions.