This is a difficult question. However, in the scriptures, the Buddha is recorded to have said "Kamma, I say, is intention"; and even in human law there is a recognition of the difference in the person's intention in deciding the penalty that they will receive. So, in human law, someone who acts in self-defense certainly does not get the same penalty as someone who initiates premeditated murder. What you would do at that time also depends on how ready you are to die at that moment; whether or not the feeling that you don't want to generate that Kammic action of killing, and would rather die with Compassion and Lovingkindness in the mind, arises.
There is a story of the Buddha in the scriptures that tells of a time when he was a Bodhisattva. He was a wandering meditator known as the "Keeper of Patience". He was murdered by a king, and while he was being murdered he practiced patience and had only good wishes for the murderer, so that was a very high level of development of course.
There is the recognition that it is the intention behind what you do that is most important. If you can stop them without killing them, then you would definitely try to do this. To injure them, rather than kill them, would definitely be a Kammically less intense intention. Certainly I cannot tell you exactly what the Kammic result would occur, because I am not a Buddha, and it is recorded that only the Buddha knows the results of Kammic actions. However, I go with the scriptures where it is intention that makes Kamma.
If you are going to stop that person from killing in order to save your life then there would still be, I believe, some negative Kamma in that intention of killing a living being; but the intention would not be premeditated murder, which would have much worse results. So I think it would be important in your mind to try to be careful with the intentions behind your actions if you are going to do that.