Video of the Shazier hit

[hitchopottimus]

Article 8: Unnecessary Roughness
There shall be no unnecessary roughness. This shall include:
(g) If a player uses any part of his helmet (including the top/crown and forehead/”hairline” parts) or
facemask to butt, spear, or ram an opponent violently or unnecessarily.


There's the rule. Unnecessary spearing, butting, or ramming with the helmet, regardless of the status of the other player, is unnecessary roughness. It's exactly what happened on that hit.