Here's the legal rubric, the "Five Elements of Self-Defense":

  1. avoidance: duty to retreat: You must attempt to retreat or avoid the confrontation if it is safe and possible to do so without increasing danger.
  2. innocence (non-aggressor): You must not be the initial aggressor or have provoked the incident.
  3. imminence: The threat must be immediate and underway (or about to begin imminently)
  4. proportionality: The force used must match the threat level without excess.
  5. reasonableness: Your belief in the necessity of deadly force must be reasonable based on the facts known to you at the time.

To me (3) seems least controversial and (1) most controversial.