Interesting discussion, i haven't read the whole thread just the debate around the charges.
My personal opinion is that either Murder or Manslaughter will be proven based upon what we know. There seems to be confusion that maliciousness is needed to prove murder ( i think it was Simon who said, though I maybe wrong). The mental element the law adopts for murder is 'malice aforethought' ... it's an antiquated legal term and doesn't equate to maliciousness. It basically means intention and that intention is based upon a foresight of consequences. That intention can also be based upon an intention to do grievous bodily harm (or the US equivalent, forgive me i'm English). If the resulting consequence is death from grievous bodily harm it is still murder, even though there wasn't an intention to kill.
The other avenue prosecutors can go down is involuntary manslaughter. As has been mentioned this an be through negligence, but it can also be proven through an assault that leads to death ... ie, i punch you on the nose, not realising that you have fragile bones and die from my punch. I did not intend to cause you death, or grievous bodily harm, i only intended to assault you. Involuntary manslaughter is death caused through another criminal act (ie my assault).
Another aspect of this case is joint enterprise or a common purpose. So for instance i'm with a gang of other guys. We all join in beating someone up. One person kicks the victim in the head, causing death. In the eyes of the law joint enterprise is used to find all guilty of murder or manslaughter. It does not matter who struck the fatal blow, the fact that we were all part of the initial beating means that we could all be guilty of the unlawful killing by association. Its a very contentious point of law, as it was initially intended to stop a group of individuals denying they struck the fatal blow and hence declaring themselves innocent. It is in need of reform, however it is still on the books in many countries, including the US.
So how do we apply this to the case at hand?
For murder the prosecution would have to prove the officers either intended to kill or intended to cause grievous bodily harm by their so called 'rough riding' and as a result Freddie Gray died. For the purposes of intention, proving that the officers had a virtually certain foresight of death or grievous bodily harm could be enough to prove the mental element (malice aforethought), also known as intention. Proving either of these would mean murder.
For involuntary manslaughter the prosecutors would have to prove that the officers assaulted Freddie Gray which eventually resulted in his death. Personally i think this has a better chance based upon what we know. As the arrest appears to be wrongful arrest, the officers would more than likely be up for an assault charge. This is enough for involuntary manslaughter, as the result was death. The death could be viewed as a continuous act from the initial assault (wrongful arrest), straight through the rough riding ... either the initial assault or the rough riding proven could be enough for an assault charge. If prosecutors proved this lesser offence, then as the final result was death involuntary manslaughter would in my opinion likely stick.
Under the concept of joint enterprise, it does not matter which officer struck the fatal blow. If they are all involved in the assault and death was the eventual result, then all officers could be charged in the same way as the one who was doing the driving of the so called 'rough ride'.
It will be an interesting case to follow and one that i will certainly follow.