What gives you the right to defend yourself or others?

The legal case for self-defense

In relatively civilized places, the legal system grants the right of self-defense — and often the right to defend others, or even protect property — to every victim of violent crime during the commission of the crime.

The moral case for self-defense

If we prohibit fallacies (like special pleading) in our reasoning, and then we postulate the following points:

• Aggression — defined for this argument as the imminent and undeserved initiation of the use of violence against anyone — is morally wrong.
• Stopping aggressors is optional, but those who do — we name them defenders — act heroically (morally virtuously).

...then it's clear that anyone using any violence necessary to stop an aggressor in the commission of aggression is morally virtuous.

These are standard (if a bit abridged) postulates of the voluntaryist ethic.

In uncivilized societies it is prohibited (either in principle or through any practical means) to use violence in defense of self, others or property; the State reserves the use of violence exclusively for its own agents (this is the fallacy of special pleading mentioned before, implemented as legal statute).  Of course, in these societies — evil by the above definition — victims of violence often go poorer, or raped, or dead; this is the first evil of such societies.  If said victims they successfully defend themselves, then the State ruins their lives; this is the second evil of such societies.  The third evil of such societies is that aggressors know their victims are likely to be defenseless, so they feel empowered to commit acts of aggression without fear of reprisal.  In this respect, the degree of evil of such societies is proportional to the degree of strictness of their prohibition of defense, as well as the degree of zealousness with which said States prosecute defenders (much to the joy of aggressors).

Needless to say, people who agree with such policies are supporters of evil themselves  — since their agreement constitutes tacit support of evildoers at the expense of defenders..  If someone says that you should not be allowed to defend yourself against someone trying to kill you — or, worse, that you are evil and should be punished if you do defend yourself — what they are saying in effect is that they want evildoers to prevail and you to die.  If that's not evil to you, then you do not know what evil is, and no one should listen to what you have to say about that.