I think it will be relevant in that the cops story hinges upon " he grabbed my taser. If he can credibly articulate that he believed the guy to be armed with a weapon he had just taken the next level of force is bang. Its less an issue of what you or i see on video or feel than what he felt and experienced.
True, the legal doctrine is based on what the shooter reasonably believed to the the facts, rather than what the facts might have been. But ... the legal doctrine is based on what the shooter
REASONABLY believed to be the facts. The video clearly shows the victim running away from the officer. What difference would it make if the officer believed that the victim had possession of the taser? Even if the victim had the officer's taser in his hand, he was making no attempt to deploy it, and he was not in any way posing a threat to the officer. He was running
away.
The officer did not look, as he started shooting, like a man in fear for his life. He looked like a man plinking targets at a shooting range on a Saturday afternoon.
If the face of the video, I don't think his "I was in fear for my life" claim is going to go anywhere.