Excessive force is determined by the State in the US, as well as firearm laws. Canadian laws are national.
Florida recently passed a law whereby if someone comes onto your property, and you feel they are a threat, you can shoot them.
http://www.nytimes.com/2006/08/07/us/07shoot.html
There is an extensive discussion about the legality of using a gun for self defense in Canada (You may have to sign up):
http://www.canadiangunnutz.com/forum...ight=house+gun
It can be done legally. Not as easily as in the US, but it can be done. But you need to familiarize yourself with the laws about keeping a legal firearm, and when you can use that firearm for self defense.