What Mehserle did was significantly worse than Frederick. Yet Mehserle gets a lesser charge with only 2-4 years. That in itself is illustrative that he got off comparatively easy because he's a cop.
I think it is a fair resolution. I have been to many train stops in large cities and they attract some really bad people, bums, and thugs. They usually all carry weapons. These are not easy patrol areas. Not an excuse but I can see where you can be on edge at these areas and make a knee jerk mistake.