He would have gotten more than 2 years if I had been on the jury.
I consider the similarity a design flaw in the sense that a moron who has no business touching either one might confuse the two weapons. I also consider the shooting a serious crime. You don't pull the trigger on any weapon if you're not certain that what you're doing is what you mean to do. BTW, I'm not convinced the cop is telling the truth here - it's at least plausible that he thought the criminal needed killing. He may even have been right in that thought, but the "oops" excuse doesn't cut it with me.
Note: The Taser has no significant recoil and is not used at long range, so it doesn't need a pistol grip and could be shaped so differently that no one could confuse the two, not even under stress and in the dark.