If you come across, two people fighting engaging in an afray, and when you come in sight of them you see one is on top of the other and you see a knife going into the one on the bottoms body, and you use deadly force, on its face you would think you would be ok for attampting to stop a murderous assualt. However when it was all over other witnesses point out that the one on the bottom started the altercation and that he had the knife and was trying to stick it in the other subject and as they rolled around the good guy finally got control of the knife. Then at that point you can be arrested for murder or assualt with a deadly weapon with the intent to kill inflicting srrious injury (if the one you shot did not die).