As long as the threat exists,you are completley justified to start the attack and kick your opponents ass(to a degree).If your attacker ceases his intention,to where he is no longer a threat to you,then you'll be going to jail if you run him down and pound him.

As for lethal force(stangling,strikes to the throat e.c.t)I think the situation is the deciding factor.Some people simply think or say "I'd rather be judged in court by twelve than be carried out by six" and with that mentality,they may apply such force unessecarily.

A general guidline...you are allowed to preserve your life by means of severe force if

(A)your being attacked by two or more people with true intent(NOT your friends throwing you into the pool or something!)
(B)being attacked with a blade(again use discretion...no need to choke your mom while she's cutting the brownies)
(C)being attacked with a firearm(this ONLY apply's if the attacker is within close range and you feel he is going to shoot you anyway when you hand him your wallet/money...otherwise its better to run like hell.)

Those are just very rough guidlines,but the situation should alway's determine the amount of force thats justified by the law.