Possibly responsible for an unsecured passenger, to some lesser degree, greatly reduced by the unsecured passenger’s contributory negligence, in a CIVIL matter.
Never in a criminal matter.
Even with much “better’ autopsy evidence, in the real world, this case will never be tried in Baltimore, and at least one of the 12 would say to themselves that Grey, the punk drug dealer, got what he deserved.
Only takes one.
If you are under arrest, the officer is completely responsible for your safety from the time of the arrest until he gets you to the station. The situated is compounded by the fact that Freddie Gray was handcuffed and shackled and unable to prevent harm to himself. As he was unable to prevent harm to himself, it became an obligation to the police to ensure his welfare.