the case you linked, while it was indeed a kill, seemed more of an accidental murder. seemed like the cop really did mistake his real gun with his tazer. doesn't excuse the killing, but I don't think the cop willfully chose to just shoot the guy in the back for no reason.
That is impossibly hard to do on accident. The difference in weight/grip between the tazer and handgun is very different, the uniform standard typically dictates where all equipment is on the body for all officers (he wouldn't reach for his handgun and pull a tazer out of the holster), the situation doesn't call for a tazer either.
That was just as fucked up as it looked.