the answer is in your statement. If use of the taser is justified by the actions of the person and the situation than Id say the use of it is justified..

put it another way with respect to illegal drugs, do the police have the right to shoot to kill someone using illegal drugs?

the drugs may be illegal but its not the death penalty.

(just like the kid in Vancouver who answered the door with a tv remote in his hand -.. the police who were at the door shot and killed him because they thought he had a phaser.) ok thats a different story..
but they went unpunished.