Go read our Common law. I have passed Criminal law and Crim Procedure and have worked on case law that discusses the technicalities of self (our law defines it as Private Defense) defense. I do agree, killing someone over a laptop will be hard to prove. It depends on the fact of the case, was the attacker wielding a weapon etc etc. Reasonable person's test will be used so the courts do adopt a subjective and objective approach.
Quick copy paste from Wikipedia, I am at work.
This defense is available when a person uses force to defend an interest in property: for example,
- to prevent a would-be thief or robber from taking his own property, or that of another;
- to prevent someone from damaging or destroying his own or another’s property; or
- to prevent an intruder from entering his own or another’s property. This would include the use of such preventive devices as spiked fences and electrified fencing.
The requirements for private defence of property are similar in many respects to those for private defence of persons, but there are certain differences. The following are conditions relating to the attack. There must be evidence that
- the property was
- presently
- in danger of damage or destruction
- that was unlawful.
The defence of property must be
- directed against the attacker;
- necessary to avert the danger; and
- a reasonable response to the attack.