I'm not posting this as an admonishment to those here who get a little carried away, the 'bloodthirsty'. I'm just posting it as a reference. 99.9% of the posters here are kind, gentle folk who just want to be left alone and to be safe. Heck, even I might get a little miffed if you attack my loved one(s). Hope this is of use. :)

UseofForce.us: AOJP

Good reference reading.

Ability
Opportunity
Jeopardy
Preclusion

Know it, live it.

(from the site)
Always remember:

The threat must be current, immediate, and unavoidable.
Your level of force must be appropriate to the threat.
Your use of force must stop when the threat ceases.

If at any point you smudge the first, exceed the second, or forget the third, you are running the risk of a criminal indictment—and if the results are glaring (e.g., you killed him), it’s nearly certain.