Neccessity

Necessity is a Complete defence in criminal law. It is essentially an argument that committing a crime was the least worst available option. An (unsuccessful) example is argued in Moss v Howdle where a this was argued as a defence to speeding on a motorway where a passenger was in great pain - the defender argued that he was speeding to a motorway rest point to provide assistance, and thus his actions were "necessary" to avoid a possible greater harm.

Although there are some cases in other common law jusristicitions on this point, there is no current Scottish case law stating if this can be used as a defence against murder.

Requirements
The requirements for the Necessary defence originate from common law and state:
 * There must be am immediate danger of great bodily harm
 * No other reasonable course of action must have been available
 * The action must have had a reasonable chance of removing the danger

Additionally, it is thought that the accused must have attempted to weigh up or consider the factors in the situation.

Immediate danger of great bodily harm
In D v Donnelly an intoxicated young lady who had been locked in her car, whilst her attackers rendered the nigh club where she had been drinking, who then started and drove the car (whilst intoxicated) was unable to use this defence as she was not in immediate danger.

No other reasonable course of action
Turning back to Moss v Howdle This defence was not successful as it was felt that the defender had alternative options than to speed to the next motorway rest point, most of the motorway network has hard shoulders, and thus stopping immediately was an option.

Returning also to D v Donnelly, this case additionally was unable to successfully use the necessary defence as the defender had other options available than just driving the car.

The action must have had a reasonable chance of removing the danger
In Lord Advocates Reference (1 of 2000), a number of trident protestors has their recent aquital reviewed; in this case it was determined that the defence of necessity failed on multiple grounds, including that their actions were not likely to have any prospect of preventing the danger.

Concious consideration of the choice
In Dawson v Dickson it was suggested that it is also required that the defender should also consider their options beforehand.