I didn't see that he was handcuffed. Even still ,handcuffed people can pose a very real threat -- but that's starting to push limits of reasonableness and I see the issue.
In the end I think elevating use of force to affect an arrest is fine, especially if the subject is the one essentially forcing that elevation. In this specific case, I'll be willing to bet the use will be deemed reasonable because the guy was still resisting -- albeit, I agree, it's distasteful.
That said: I am one of the Taser instructors for my departments and this is exactly the type of scenario I train my personnel to avoid. In my opinion, and what I train, is that a Taser is a device used to help gain control in a potentially, or actively, violent scenario. This is specifically why we have a no-use on passive resistors policy.
Again... this is one of those scenarios that looking back -- from what I can see -- looks like it could have been handled differently. Also, remember, we're not there and we don't know the *entire* story, so I'm going off what we can see. Like, is that cop alone? How has the guys anger / emotion increased over time? How big is he? How long until cover arrives? Etc. All these things come into account.