That's often why victims lose. Your basic problem, if authorities examine the evidence, is to show that your life was in danger. If you were faced with a knife or other deadly weapon, your fear was certainly justified. Therefore your actions will be likewise justified. It doesn't matter where the knife came from.
Forensic science is one area of the law which has improved tremendously.
Some of the best police work in the country is now done in the lab. I've heard statements by people who want to defend their homes and are half paranoid due to the high crime rate. Any hunter knows that you can't move a bleeding animal without leaving a blood trail a blind man could follow. Even without a blood trail, footprints outside your house will reveal just how the incident took place. On the other hand, if the person lying on the ground outside your house also had a gun lying there with his prints on it, it is more logical to believe you were afraid he would shoot at you through the window if you didn't shoot first.