I ran into something similar as well.
This event (pilgrimage.6750) is clearly written as if it is about one of your *own* secrets, but what it actually does is it takes a secret you *know* about someone *else*, and you have the option to remove that other person's secret for stress. I your case, one of the murderers you know would no longer be a murderer because you repented for them.
...My personal opinion: Even if this event correctly removed your *own* secrets, it would not really make sense. You are not suddenly "not a murderer" or "not an adulterer" because you repented. It would make more sense for things like deviancy, witchcraft, etc., but even then other people who already know your secret would still have the corresponding evidence.