This one's a hard one, the way I see it, and a good example of where football is right now.
By the rulebook, legal. There was helmet to helmet, but the blocker clearly led with his shoulder. Simple physics and physiology that the shoulder is close to the head and at those speeds, things keep moving.
However, with the defender having no clue that the blocker was coming, the same goal (block to prevent a tackle) could have been accomplished with a same speed push, leading with the hands. In my mind, an instance of 'ohhhhh....ima light this effer up right here'.
But how do you deal with that? If a hard hit, just because it's a hard hit, is illegal, then we're playing a different game. On the flip side, hits like this are why the NFL is getting sued by former players for brain trauma.
You could take the easy path and say let the refs decide...give them more leeway on 'unnecessary roughness', but we're already arguing those calls today, giving them the reigns to decide what's justified and not is only going to get more controversial.