There used to be a rule that, before calling a KOS, you had to ask them for the reason as to why they committed these acts. There are two reasons why that doesn't work though. For one, the player asking the other user why they did something traitorous can choose to completely ignore their reason. The rule did not state that they also had to take their reason into account in any way, only that they had to ask for it.
The other reason why we removed the rule was because it slows the game down unnecessarily. TTT is a game about speed and strategy - if you're too slow, you die. Asking other players for reasons does not help in any way, and only slows the game down. It does not provide anything constructive.
If you kill an innocent that decides to shoot you for no reason then there is definitely a possibility that you will die. And although you might have killed in self-defense, that does not make it any different. If we add a rule to prevent this we'll come back to the issues earlier stated - what is a good reason to defend yourself from accusations, when you've just killed an innocent? Why should we even trust you? You could be a traitor just as easily, and there's no reason to let you go other than the possibility of a karma hit.