There is often a fine line between defending yourself and committing a crime. It is very easy to misjudge a situation and end up with charges of assault or even homicide against you in a situation where you felt you were just defending yourself. The law explains specifically what circumstances must be present to allow you to claim self-defense.
According to the Arizona State Legislature, self-defense is a viable excuse if you are in a situation where you feel physically threatened. The other person needs to be physically assaulting you or attempting to when you use force against him or her. The other person saying he or she will hurt you is not enough to justify the use of force against that person in self-defense.
If the other person leaves the situation once you begin to defend yourself, you cannot go after that person and continue the assault. This falls outside the definition of self-defense. Do note that you do not have to try to get away before using force against the other person. Some states do require you to try to retreat, but Arizona is not one of them.
You need to feel that using physical force is the only way to protect yourself. When a court looks at the situation, it will determine if any reasonable person would have felt the same way. If it determines that anyone else would have acted as you did, then it falls under justifiable self-defense. However, if the court believes that other people would not have taken the same actions, then it may find it was not self-defense under the law.