This is an interesting hypothetical situation.
Assuming I can't physically stop the man, I would let him kill as many people as he damn well felt like. Each day/increment of time he went out to do his killing, I would attempt to contact police, to get myself arrested as his accomplice. I would then, assuming that both of us were captured, (this lunatic is clearly fixated on me, and this is a weakness to be exploited,) I would cut a deal with the prosecution for a reduced sentence in exchange for information that would put the killer in prison for life without parole, or put him on death row. The list of murders he performed to "keep me alive" would serve to cement his guilt.
I will no doubt have to live the rest of my life with the stigmata of a man who "let others die so he could live," but I would still be alive at the end of it.