Self defense does not imply eye for an eye. If someone shoots a thief breaking and entering their house, they have acted in self defense -- even if that thief only intended to steal property. If kidnapping only carried a prison sentence equal to the length of the kidnapping, it would be no deterrent. You think OJ's going to worry about less than a day in jail? Furthermore, jailtime is not the same as being held by a bunch of thugs with guns who could wheel off and shoot you at any point.
I don't know if your question was in earnest, but the short answer is, self defense and justice are not limited to eye-for-an-eye. If you act violently against someone you are at the mercy of the circumstances in how they defend themselves, and later at the mercy of what the people have determined is a reasonable punishment for the crime you have committed. Neither the person defending themselves, nor the justice system is in this case initiating violence -- OJ initiated the violence.
If you wish to hold a strict eye-for-an-eye view of justice (or even moral self defense), you are welcome to it, but that is not libertarianism, and I would certainly disagree with you.