I don't know about the law in your state but here in NSW, Australia, the Crimes Act 1900 states:
Quote:
418 Self-defence—when available
(1) A person is not criminally responsible for an offence if the person carries out the conduct constituting the offence in self-defence.
(2) A person carries out conduct in self-defence if and only if the person believes the conduct is necessary:
(a) to defend himself or herself or another person, or
(b) to prevent or terminate the unlawful deprivation of his or her liberty or the liberty of another person...
and the conduct is a reasonable response in the circumstances as he or she perceives them.
421 Self-defence—excessive force that inflicts death
(1) This section applies if:
(a) the person uses force that involves the infliction of death, and
(b) the conduct is not a reasonable response in the circumstances as he or she perceives them, but the person believes the conduct is necessary:
(c) to defend himself or herself or another person, or
(d) to prevent or terminate the unlawful deprivation of his or her liberty or the liberty of another person.
(2) The person is not criminally responsible for murder but, on a trial for murder, the person is to be found guilty of manslaughter if the person is otherwise criminally responsible for manslaughter.
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Of course, the law WILL differ (maybe only a little) in your state/country, but generally you do have a legal defense of self defence, which (despite the word 'self') does include the defense of another.
You MAY be charged for murder, in which case you can claim you were acting in self defense and then the onus of proof is on the prosecution (ie: the State) to prove that your actions were NOT self defence. I'm not sure what the cops would do upon arriving at the scene, but as always the best thing YOU could do would be cooperate with the police, voluntarily return to the station and make a statement, etc.