Well in Garner SCOTUS specifically said:

Thus, if the suspect threatens the officer with a weapon or there is probable cause to believe that he has committed a crime involving the infliction or threatened infliction of serious physical harm, deadly force may be used if necessary to prevent escape, and if, where feasible, some warning has been given.

So in this case it would be justified as soon as he threatened the officer with a weapon (which he did as soon as he attacked them and stole the taser) and the justification wouldn't disappear if he dropped the taser

Unless for some reason the court said that a taser was not a "weapon" but this would be a silly decision that seems unlikely