I put myself in this officer's shoes. I have a guy who first got out of the car when he shouldn't, doesn't return to it when told to. Then he starts walking away when told not to. When grabbed by shoulder and told "hey, get back here!" then runs into a yard. I suppose at this point the cop *could* have just said eff it and detained the second individual and secured the vehicle, but there is probably some responsibility to not let a suspect loose in a residential neighborhood, because one has to wonder why he's running away from a very simple traffic stop -- is he wanted? Is he carrying?
So the cop makes the choice to try and continue with his effort to detain the suspect. At this point the suspect now escalates the situation by fighting and resisting. So the cop decides to respond by deploying a non-lethal weapon, his taser. At this point, the suspect then GRABS the taser. Now the cop is in a really bad situation -- if he loses control of the taser, and he is the only officer on the scene, what would prevent the suspect from using the taser to incapacitate the cop, at which point the suspect could beat him, stomp him, keep tasing him or grab his service weapon and shoot him? At that point, I don't think the officer has much choice other than being the one to use the service weapon.
I see zero chance this shooting isn't declared 100% justifiable. That doesn't make this a non-tragedy, however.