If you get tased, it's not like TV where the person comedically stands there for a few seconds, makes a some funny noises, and then carefully drops to the ground. The officer being tased in this situation could have resulted in any number of serious injuries based on where they were tased, and the how they collided with the ground afterwards.

This guy proved beyond a shadow of a doubt that he was violent and dangerous. After pointing the taser at the officer, there's no reason to suspect that he wouldn't have seriously harmed the officer with it if given the chance.