The defence of self defence is available to crimes involving the use of, or threat of force. It can arise in circumstances involving murder and even assault and will result in a complete acquittal.
There are not a lot of rules limiting its scope and it is a generally a matter of fact for the judge or jury to decide.
Under section 15 of the Criminal Law Consolidation Act it is a defence if the accused genuinely believed their conduct was necessary and reasonable for a defensive purpose; and the conduct was reasonably proportionate to the threat that the defendant believed to existed.
If the defendant reasonably believed that their conduct was necessary and reasonable for a defensive purpose but the conduct was not proportionate the it can be a partial defence to murder and not a complete defence as in the other scenario. It will reduce the offence to manslaughter if section 15 (2) applies.
The test as to whether a person “genuinely believed” is subjective. The belief must be laid down on reasonable grounds.
The courts have held that evidence of the victim’s reputation, past convictions of violence and past acts of violence are admissible as evidence to show the victims tendency to be violent or to show they were the aggressor
Burden of Proof
The prosecution have the burden to negate and reasonable possibility of self defence, however only when the issue of self defence has been raised by the evidence. The accused however has the evidentiary burden.
It does not require that the defendant give evidence of self defence it is only necessary for the whole of the evidence to show that a reasonable possibility exist. This means that even if the defence has not raised self defence if the evidence suggest a possibility then the judge must direct the jury to that fact.