How could you argue that the assassination of Franz Ferdinand was justifiable?
To argue this, you would have to say that there is an inherent human right to be ruled by people of your own ethnic group. You would have to say that this is a right on the same level as the right to life. If there is such a right, then you have the right to kill those who try to take that right away from you. The Archduke was, in a sense, doing that since he was a German who was one of the people who ruled over a group of Serbs. If you make all of the above arguments (which seems pretty difficult to do) you can argue that killing Franz Ferdinand was acceptable as a way of defending basic human rights.