Overall the Mehserle jury probably got it right. Second degree murder, and voluntary manslaughter, both require proof beyond a reasonable doubt that Mehserle had a specific intention to shoot Grant. The evidence at trial offered reasonable doubt that he intended to shoot Grant with his gun (the "oh shit" that several witnesses read on Mehserle's lips just after the gun went off speaks to that). Had the jury decided to acquit or been unable to reach a verdict (as was the case in the Oakland Riders case several years ago), the outcome would have been far more devastating in the message it would have sent to police officers. As it is, even the minimum 5 years in prison which Mehserle now faces will be a powerful deterrent. Police know what prison time means (and have lots of good reasons to fear it more than those they typically arrest). Even more importantly this verdict and the massive civil judgment that Grant's family is likely to obtain against the BART police force will influence police training in the management of conflicts and the use of Tasers.
What we need to do now is move beyond the focus on what Mehserle did and focus on the underlying police policies and practices involved. What the trial revealed were a group of arrogant police officers who believed that BART was a throne from which they ruled over anyone who came to their attention. The behavior of Oscar Grant and others on the BART train that night was not exemplary but it was hardly exceptional given that it was New Year's Eve. It is not clear that there ever should have been an effort to remove Grant and his friends from the train (if the goal was moving people home fast and safely, placing a police officer on the car in question for the duration of its journey would have been a better move). The decision to handcuff them was clearly taken to punish Grant and his friends for being disrespectful and boisterous. That the officers involved were fired is a starting point (and any effort to reinstate them should be met by massive protests), but as in Abu Ghraib, misconduct by line officers never happens without a culture that tolerates or encourages abuse from the top.
We should start with some basic questions. Why do BART police need Tasers let alone guns? I haven't found the statistics on BART crime yet, but I would bet that overall the system compares favorably in terms of crime with an average Middle School. Indeed, why do we need a BART police force at all (the actions of which are likely to cost us millions in civil suits)? A uniformed conductor service (such as those found on Amtrack), equipped with radios to summon local police should suffice to resolve the vast majority of situations.
Nothing can give Oscar Grant back to his family (and overall the effort to give victims "closure" through punishment is probably misguided). They should receive massive civil compensation to help them cope with the practical consequences and to provide a measure of deterrent protection to the rest of us. What we need to do now is demand institutional changes in how security is handled on BART going forward.