Here is what the California Penal code says of PC 187 - Murder is the unlawful killing of a human being, or a fetus, with malice aforethought.
That means Mehserle purposely took out his gun and meant to shoot Grant in back in front of everyone.
The murder charge was political and only brought to calm everyone down.
I don't know if Mehserle can be convicted of a lesser charge. From everything I know at best he should have been charged with Negligent Homicide. Meaning he didn't mean to do it but because he was poorly trained and was probably someone who shouldn't have been an officer he made a mistake. That's unfortunate but it happens and life sucks.
The Grant family should sue and take the money they get which will probable be in the millions.
Being a police officer is a hard job and its not for everyone. Unfortunately some morons get past the process and end up on the streets. I work with a couple of them and keep a sharp eye on them when I'm around. It's only a matter of time until they do something stupid that will get my City sued or someone killed.
So everyone get ready for the riots in Oakland sometime later this week. Luckily I'll be out of town but if the riots go until Sunday and Monday I'll be there trying to keep the peace.