There are lots of variables that can affect this. How recently the murder happened, if it was a random killing or domestic issue, how grisly the murder was, etc. Sometimes if the market is booming and renters are having trouble finding homes, you won't get much of a discount at all.
hahahahahaha at the answers, I used to think that realtors were stuffy, now I find they have a killer sense of humor. I read of a story of some woman who suddenly discovered that she had bought a house where a murder occured. Apparently she was able to sue the realtor for not disclosing the house past. I think homes near cemeteries might have lower rent if you are looking to lower your rent.