Cant speak for the US, but from a UK standpoint (and this makes the whole situation suck even more because it didnt seem to work for the guy who's going to be doing time soon) It's all about the point of "reasonable belief" in court.
For example:
Your belief that you (or a family member, vulnerable person etc) are in danger of "serious bodily harm or threat to your life" from an assailant. You have to be able to show that you had reasonable, realistic, and applicable grounds for this belief. Such as:
1 You belief the guy had been drinking, you had "reasonable, realistic, and applicable" reason to believe this because he stank of alcohol, was slurring his speech, was waking unsteadily etc.
2 You also believe that you/family member, other person was in significant risk of harm. You had "reasonable, realistic, and applicable" reason to believe this because the person was making verbal and physical threats etc, helps if people witness this.
3 You believe person was under the influence of drugs, see reasons for under the influence of alcohol.
4 You believe person had a weapon, this is an interesting one as in test-cases its been seen that the assailant "technically" does not HAVE to have a weapon for you to take "appropriate" action, its all about what you BELIEVE but you also need reasons to support that beliefe, even if you are wrong, such as the assailant put a hand in his pocket/behind his back, and kept it there.
5 Assailant's physical size/percieved ability (I've used this myself after being arrested and interviewd for breaking someone's jaw while defending myself as I'm really short. 5"5 ish).. The assailant is physically much larger than the person defending themselves, has a reputation around the area for being particularly aggressive, being a boxer, doorman, etc..
Basically, in your defense if you can testify to all or a combination of the above, you SHOULD be ok and found not guilty of commiting assault (I say "should" because due to a recent thread on here it shows that the above doesnt always work)
Unfortunately, knowledge of how the system works before you're unfortunately put in a position whereby you have to use it, can often be the difference between you being found "guilty" or "not guilty" of assault or something similar. But then I could easily say that having this knowledge should be your brief's responsibility (solicitor, lawyer etc). Unfortunately as seems to be the case of most things, some legal professional's are better than others. Again unfortunately it's often the fact that the more expensive a brief, (and the more inaccessable to most people) the better and more effective he/she actually is.
At the end of the day though as recent events have shown, the above is all pretty much just words. Which is bollocks when it doesnt work for you.