If you mean what constitutes just cause in the eyes of the law, there are still widely divergent standards. Under TX law protection of property with deadly force is a defense for homicide, in most states it isn't. I believe the laws of all states deem self defense to be just cause, but the threat level to life and limb that must exist for a homicide to qualify as self defense varies.
In the final analysis just cause is usually determined by a jury. One set of jurors may be persuaded that a defendent had just cause for homicide, another jury presented with the same evidence may not. Was the U.S. justified in bombing German and Japanese civilians apart from sites with military value during WWII? Ask 100 people at ramdom and I guarantee you will not find a consensus on that issue.