There is a difference between accidental and intentional homicide. I have no idea what actually happened here, but saying they imposed a death sentence implies they intentionally killed him.
Agreed nobody does (including the protesters and rioters) until we hear testimony. I will admit it doesn't look good but lets see if there is a logical reasonable explanation and if not then convict the officers that did wrong.
Actually there is a continuum from intentional homicide, through negligent homicide (which includes foreseeable accidents which those having custody of a person are responsible for taking measures to prevent) to accidental death for which those who having custody of the person cannot be considered legally responsible because it arose from circumstances which could not reasonably be foreseen.
Even death resulting from negligence on the part of the police while in police custody is culpable to the point I do not regard my rhetoric as extreme. We do know Mr. Gray died of a severed spinal cord. This is not something that happens without extreme violent trauma. Absent a vehicular accident caused by someone other than a police driver, a freak accident involving slipping and falling onto a sharp edge or projection, or a sudden attack by another prisoner — none of which seems to apply in this case — I cannot conceive of any circumstance which would subject a prisoner in police custody violent trauma severing his spinal cord, for which the police would not rightly be held culpable.