I don't have strong feelings on the verdict of this case. I haven't been following the case in detail. I understand the defense claimed Mehserle meant to grab and use his other potentially lethal weapon while Grant was face down on the ground. Mistakes can be made very easily in high stress situations. Perhaps it is not unreasonable to give the benefit of the doubt in such conditions.
However, benefit of the doubt in uncertain, high stress conditions should be given to more than just police officers. Another example of uncertainty and high stress that could possibly lead to honest mistakes is when police bust down a door unannounced in the middle of the night, shooting every dog in sight. A sleepy resident might be confused, frightened, and unprepared as to the best way to react, possibly leading to violence by mistake. But non-police are not always given the same benefit of the doubt.
If anything, the police should be held to a higher standard, given their training and their status as bearers of the public trust. But really, it would just be nice if the law applied to police and non-police civilians with the same presumptions and the same force.