I think it's the case that the prosecution aren't disputing the fact that the decision to use the taser was justified, it's that they shocked him for way too long, and then kicked him twice. I'm not a legal expert but I'd be surprised if the murder charge stood up, but would also be surprised if the manslaughter charge didn't. If you kick someone in the head who is unconscious on the ground and they die, there's not really much of a defence to that is there, especially if you're a trained police officer.
Shortest prison sentence on record incoming.