The first part is idiotic, but the second is completely reasonable. Here's what the law states in Canada:

Defence — use or threat of force

34 (1) A person is not guilty of an offence if

(a) they believe on reasonable grounds that force is being used against them or another person or that a threat of force is being made against them or another person;

(b) the act that constitutes the offence is committed for the purpose of defending or protecting themselves or the other person from that use or threat of force; and

(c) the act committed is reasonable in the circumstances.

If someone is trying to kill you and you have a gun, I think you'd have a strong defense for killing them. However, if they obviously don't have a deadly weapon, you'd have a much weaker defense.