This is taken from an article written quite some time ago. I used it as part of my Black Belt Thesis - The Legality of hand to hand self defense. I would say a lot of these principles apply to armed self defense as well.
It would be foolish to try to rely on a general understanding of the
legal principles at work in these situations, in order to engage in
behavior which falls just within the realm of legality. Rather, the wise
martial artist will attempt to avoid any hint of liability or criminal
conduct. The following general principles may be of value in this endeavor.
• Avoid physical confrontation. If there is a safe avenue of
retreat, use it (regardless of jurisdiction). At a minimum, retreat to
the wall.
• If confrontation is inevitable, give a warning when defending
property, unless doing so would be dangerous or futile (which is often
the case). This does not mean that you should list your qualifications,
as the samurai of old were wont to do. Rather, you should simply give
the aggressor notice that you intend to use force against him, in order
to allow him to reconsider his position.
• Ensure that you are not seen as the aggressor. This does not
require ‘taking the first hit’, but it does require being certain that
physical contact is imminent prior to reacting (for an in-depth
examination of the danger here, see the Goetz case).
• Be aware of the aggravating and mitigating factors. Is there a
size, age, or ability differential? Are you or the attacker armed or
trained? All of these factors will help you determine the appropriate
level of force.
• Use only the amount of force necessary to deter the attack. This
does not require the use of ineffective technique, but rather mature
reflection prior to a confrontation about what technique (including
flight) is appropriate in which situation. It would be wise to introduce
this as part of training.
• Once the initial threat is neutralized, stop. This does not mean
that you must give your opponent a fighting chance. Rather, you may
immobilize the attacker while awaiting the police, but do no further damage.
• When intervening on behalf of a third party, ensure (as much as
possible) that the intervention is justified and necessary. As a rule,
interference in domestic disputes is unwise. Reconciliations can mean
trouble for the would-be rescuer.
• Remember that, in this country, human rights are superior to
property rights. The use of force in the protection of property is very
risky.
• As an instructor, you are both morally and legally responsible for
the actions of your students, both inside and out of the dojo.