Is self defence Illegal? Do you have to wait for the other person to shoot at you first, if he is acting in a threatening manner towards you in the first place?
Since we are talking "Criminality" here rather than "War". What are the laws regarding Terroristic Threatening?
If someone is threatening you, do you have to wait until that someone actually harms you before you can react? Is that reasonable? If you do it may be too late before you get the chance to react. If someone is pointing a gun at you are you going to wait for him to pull the trigger before you use your gun? In most law, at least in more rural states, the act of posing a threat, if you reasonablely believe that you are in danger, that the threat is real, that you may lose your life if you delay, that is sufficent justifiable cause to act in one's own self defence.
In the case of Fort Sumter, after months of negociations failed to reach a peaceful solution, Lincoln made it quite clear that it was his intention to resupply and reenforce the garrison at Fort Sumter, and hold control over Charleston Harbor, to force South Carolina into submission. That was a direct threat to South Carolina's national existance as an independent state.