I'm with Shaggie on this one. The rational thinking person without violent tendencies doesn't go out of his way to get a sword and investigate. Regardless of self-defense or not (and really, it is possible that he just attacked the thief without additional provocation, since there are no external witnesses), a normal, mentally healthy person in today's society, technically speaking, does not do that. This speaks to some kind of problem with the kid, and he at least needs some psychological attention.
Also, if you're untrained with a sword, it's pretty hard to cut someone's hand off intentionally. Odds are he just sort of lashed out and the guy instinctively threw up his hand in defense.
The bottom line has nothing to do with excessive force, but that particular clause is basically outlining proportionality. If the opposition does not pose an actual threat to your life, and instead may be harming your property, it's sufficient to subdue the threat- anything beyond that would be considered "excessive." Remember, we don't have the death penalty for petty theft, and we're one of the only two industrialized countries in the world that practices it in the first place.
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NightSwan also said that she wanted to peg me, once, but I'm not sure whether to take that as a compliment or a threat...