He was restrained by handcuff that he put on him prior to walking over and picking up what you assume was the taser.
And moving it into the view of the Officers on the scene would be securing it as the suspect was restrained and not moving.
The taser can continued to be used as a drive-stun device.
As there is much that isn't known, for all you know the suspect may have fired the taser at the Officer. If that is the case the Officer shooting the suspect is justifiable.
Have you figured out that you have been wrong about the taser yet?