Legally, usually. In real life, your assumption your (presumed) thief is one of the "vast majority" is a foolish gamble. Most of the time you'll be correct. It only takes one time for you to be wrong to have something happen to you or your family. That doesn't mean my first action would be shooting wildly at them, even if I owned a gun - I'd be getting my family off the property and calling the police. But no way on Gods Earth am I ever going to assume they would never hurt me.
Maybe you should read up about it then, most states have it in one form or another. It doesn't mean you can blow away the mailman, or a thief running out the door with your TV set. But if you have a reasonable fear of your life or your family is in danger, you can use deadly force without fear of prosecution. That's as I understand it, I'm not a US resident so someone may be able to chime in and confirm.
No, of course it doesn't give you that right, nor should it.
If the thief carried a gun or a knife or a baseball bat, then you'd probably have justification. If you shot them climbing out a window with your wallet, no.
That's the thing. What to you in Canada and I in New Zealand seems like paranoid fear, probably isn't in South Africa or the US. If they're living behind high walls, barbed wire and own guns to protect themselves, then I think the law should at least not hamper a law-abiding citizen should someone find themselves up against a home invader, and should not unnecessarily punish them for their action afterwards.
That said, you don't want to give license to the closet Dirty Harry type, which clearly Pistorius is. But I'd hope to think these guys were in the minority, and the majority are rational people.