Turnabout fair play?

Radley Balko has a question about the BART shooting:

That said, there seems to be a mob-fueled rush to pin a murder charge on this guy. Given the videos, it just doesn't seem warranted to me. Speaking as a journalist who has reported on plenty of aggravating stories where bad cops got off scot-free, Mehserle shouldn't have to suffer the accumulated anger of all of those stories. He should be charged for what he did, nothing more.

At the same time, I'd pose this question to the Mehserle defenders I've seen on police forums and bulletin boards: I'm sympathetic to the argument that in the heat of the moment, Mehserle inadvertently reached for the wrong weapon. But Mehserle had training. He had other cops there backing him up. If we're going to be sympathetic to him, where's the sympathy for people like Cory Maye or Ryan Frederick?

Why should we assume good intentions when a cop with training, wide awake and conscious, with other cops all around him makes a mistake that ends with a fatality, but assume the worst when a civilian is awoken by the sound of police breaking into his home, and in the heat of the moment, fires a gun after mistaking them for criminal intruders?

Seems to me you can't simultaneously argue that trained police officers should be forgiven for nervous mistakes made in the heat of the moment, but ordinary people should be expected to show impeccable judgment and restraint, even under unimaginably volatile and confrontational circumstances.