Using a taser in circumstances when the only other option would be to shoot the target person with a gun seems, at least in principle, justifiable,

In any other circumstances it ought to be treated as an act of attempted murder. And if the target died, as an act of murder. Whoever does it - police officer or bank robber.

That doesn't seem to be how their use is regulated in places where they are in use, and I get worried about the proposed arming of police in England with these dangerous toys. I think there are some members of the force who are quite capable of this kind of thing.
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It's a bit nasty the way this thread has been dominated by a rather squalid little squabble between neighbours in North America. THe fact that this poor bloke died seems to be seen as trivial.   

This news story link has the video showing the police killing Robert Dziekanski. Pretty clearly this wasn't a situation where the only other option would have been to shoot him with a gun. Hell, there were four large policemen around him, and no members of the public anywhere near.

What is shown in that video is brutal, cowardly and incompetant, and that would have been just as true whatever country it happened in.