Motorcop has a great post on the conviction of Mehserle.
Reading the definitions of the charges he faced, there's no legal way Mehserle should have been convicted of a greater charge.
And you know, I don't hold Mehserle's strange conduct in the days after the shooting against him (mind you, I also don't hold his actions as a shining example of what to do). How should somebody react after making a horrible mistake and killing a person? I have no idea.
I don't know what I would do. I would think about my job, my life, and my freedom. And as a cop, I would also know that no good comes from talking without a lawyer. I don't say this to get sympathy for the guy, but it's more than an understatement to say it was a very tough time in his life. I think it's very safe to say, in hindsight, that Mehserle doesn't seem to perform at his best under stress and pressure.