Your first thought is, Oh man, he's dead. His head came off or his brains are scrambled and Heath Miller is trying to cover up the on-field camera because he's dead. And then your second thought is, What do you mean it's not a touchdown? His limp, vacated body totally collapsed across the plane! Football is a strange, conflicting sport for a viewer.
Le'Veon Bell appeared to score the touchdown that would give the Steelers a chance at a game-tying two-point conversion, but as he was hit by three Ravens, his helmet popped off and he went directly into the Fencing Response, a reflex action that indicates at least a moderate brain injury. (The scariest part might have been his head snapping back as he hit the ground, but Bell was already unconscious by then.)
After a replay review, during which Bell somehow walked off the field (unsteadily, with help—the Steelers said he suffered "a possible concussion"), officials ruled the ball placed at the half-yard line. Bell was injured for nothing, which is a pretty damn cynical way to look at a scary injury to a 21-year-old kid. Pittsburgh found the end zone two plays later, just reinforcing the pointlessness of it all.
The call was made correctly. Rule 7.2.1(r) is crystal clear. "An official shall declare the ball dead and the down ended when a runner's helmet comes completely off." It was instituted before the 2010 season to protect ballcarriers from continuing to run and becoming targets after their helmets come off. "We watched some tape where players are running in the field of play without helmets,'' then-chair of the NFL Competition Committee Rich McKay said at the time. "In our mind, that is not a safe situation.''
The intent is unassailable, but the enforcement is suspect. In Bell's case, his helmet came off in the process of being tackled. He wasn't made safer by the play retroactively being declared dead as he was going to the ground. Bell would have been given his continuation had he lost his helmet on any play that wasn't automatically reviewable, so why not here? All the rule did in this instance was penalize the Steelers for their runner being hit helmet-to-helmet.
And right about here is where you realize that one of the more instantly horrifying NFL moments in years has been reduced to a debate on rule minutiae that's come into play just once in four seasons, and you again think that football can be a seriously fucked up sport sometimes.