Unless I'm missing something there would not be a case to reopen. Both the killer and the victim died, unless what you are really talking about is to have a police investigation reopened because of evidence indicating it was a double homicide. So if that is what you are intending by your question it would make sense to bring the new information to the attention of the investigating law enforcement agency.
I agree with my colleague -- clearly the deceased are not subject to prosecution so who's left?
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.