You are conflating civil law with international law. In a civil context, my right to self defense is very weak. If my neighbor is stockpiling weapons, and announcing his intention to kill me, I am obligated to trust the deterrent of the state. The state has the monopoly on force. I could easily be killed by this person, but they would be prosecuted by the state, and this system defends me quite well. It is only when my life is in active danger that I am allowed to take proactive measures. If the person shoots me in the leg and drops his gun and starts running away. I'm not allowed to pick it up and kill him in response. That's the state's job (There's no chance I would vote to convict this man if I was on a jury to be frank, but that's the principle).

This is very different from the interstate case. If a state shoots another state in the leg and runs away, the only way to establish a deterrent is to shoot back. There is no police force to do it for me. This is the main diffference between civil law and international law.