In the Canadian criminal code, the rules regarding defense of person or property continuously use the phrase no more force than is necessary, when referring to how one is permitted to defend themselves or their property from an aggressor. I am wondering what exactly this means in practical terms.

For example, I am training in a martial art, and although I never intend to use force against someone unless I have to, I also intend to use force to protect myself or the people around me. The Canadian criminal code gives the impression that you cannot 'cause bodily harm' to someone, even if they have broken into your house, or are grabbing your wallet, unless they also attack you physically.

So my question is this: if someone breaks into my house and is creeping around, grabbing anything valuable, am I lawfully permitted to attack him as he rounds a corner, causing harm or possibly even killing him?

Now I understand that it is unnecessary to kill someone immediately, without at least making your intent clear, but tactically, it is important to account for any eventuality. If I simply grab his arm as he tries to run off with my property, and he turns around with a gun pointed at me, then I might end up dead. However, if I quickly break his neck before he even sees me, and before I know his threat level, have I over used force?

I am also wondering if anyone knows if these rules are different in the united states, or anywhere else, and if so, in what way?

it differs from place to place in the US
in the town i live in , if you are on my property after dark without my consent and you have a gun, you are toast.
I know that in some other places , the law actually requires you to flee your home if a criminal invades it .

Where I live in West Virginia, there is the "Three Hits Law" (Thats just what I call it). Apparently, if someone hits you three times without you striking back, it counts as assult and battery and you are allowed to defend yourself by breaking limbs. However, if someone breaks (key word) into your house, you can do whatever you feel necessary to subdue them. I know one man who shot an armed burglar through the stomach, severing three intestinal links and striking his spine. This man wasn't using a gun, he was using a hunting arrow and a compound bow. The arrow pinned the burglar to the wall while the man called 911.

West Virginia has a lot of cool laws. If you hit something on the road with your car, you are allowed to take it home and eat it (unless it is endangered). I have always wondered if this applies to humans too.

well we're definitely not endangered, anyway.

I guess that's another question. What if the robber has a knife, therefore threatening you with a weapon, and then you kill him with a sword? Is that necessary force, or is that jail time?

A law should be passed that anyone who wants to study martial arts or carry a weapon should be made to know the laws in his state regarding lethal force. I dont know maybe in some places that is already the case.

that is in fact exactly why I was reading the criminal code. I truly hope I never have to attack someone, but I want to know what the rules of engagement are if I ever do.

Part of the problem is that this martial art, and strategy in general dictate very different rules than that of society. For example, you never wait for a hostile to attack, you take him out with surprise and with lethal force.