As a sentence? Nope, no one outside the military uses firing squads any more, AFAIK, and they haven't used it in a long time. Even one of the "cleaner" methods of capitol punishment is highly unlikely for "just" kidnapping, even under these circumstances. I don't think Canada even has the death penalty for treason.

Now, if you are asking if you can use lethal force to prevent the commission of a felony, in most states here. Not sure about Canada, but most civilized nations seem to be OK with the use of force to prevent the battery or sexual assault of another, or arson of an occupied building, so I'm thinking kidnapping of a young child would fit into there. Again, not sure about Canada, and not even willing to guess on the UK, but....