Just a couple of clarifications, bearing in mind that one needs to know the law in the jurisdiction where they live. It may be different from what I say. First, as stated, if you start a fight, striking the person you start the fight with, that becomes assault and battery. You can indeed be charged with that offense, regardless of anything else that happens. If for whatever reason you decide to withdraw from the fight, and clearly demonstrate that vocally and/or by attempting to withdraw, but the original 'victim' now continues to fight, that person has now become the assailant. If he puts the original assailant (you) in fear of his (your) life, the law allows one to defend their life by whatever means is needed to overcome the person assaulting them (you). If both persons are using their fists to begin with, the second person is going to have to show justification for using a weapon. OK, so now you are charged with homicide and are in court. The prosecution must indeed prove you guilty beyond a reasonable doubt. Until you decide to claim self defense. Self defense is an affirmative defense. It means you are going to admit you killed the person, but attempt to show that you had a right to defend yourself in a manner that resulted in the death of the dead person. That opens you up to having to testify under oath, and admit under oath that you killed the person. Of course that ups the ante. So the fact you killed the person is no longer in doubt. Now you must prove you were required to use the force you did to save yourself. The burden of proof is now on you to prove you are innocent by reason of self defense. EDIT: Naturally, the prosecution is going to try to show you didn't have reason to act as you did, meaning you were not justified in killing the person. But once you assert self defense, you have eased the burden on the prosecutor. He doesn't have to prove beyond a reasonable doubt that you killed the person. Only that you lacked legal justification for doing so. In the situation given by the OP, you have no witnesses. If the prosecution can show a long history of you and the dead person arguing about something, or many things, perhaps fighting, and if you have told witnesses you want to kill the dead person, naturally you aren't going to look good in the eyes of a jury. Even if that is not the way it happened, if for instance the dead person simply angered you and you decided to strike first, then found yourself in fear of your life, what if you are known to be a person who likes to fight, and has a reputation of liking to hurt people you fight? Again, you won't look so innocent. But of course none of those things were part of the supposition from the OP. Nonetheless, as the original instigator, your burden of proof will probably be higher in the jury's mind. Another minor clarification, if you kill in defense of your family, being able to articulate a good reason for fearing for their life, it is not self defense. It will likely be considered justifiable homicide, as you have a legal right to defend your family from deadly force, and can use deadly force to do it. But it isn't self defense unless you were also under attack. In fact, you may remember Bill correctly making that same point in another thread some time ago.