One could say he cast his ballot when he cast his hand grenade; but can we say he was acting as a citizen of Canada when he did it?

He's a citizen of Canada whether he acted as one or not. I may disagree with a large portion of Canadian legislation, but I'm still subject to it (and presumably protected under it).

At any rate, we don't offer children the choice between ballots and hand grenades for a good reason—most aren't mature enough to handle either.