killing pensioners for a living

Britain has at last got its own hardcore serial killer, thanks to the astonishingly arrogant Harold Shipman.
His story is perfect Horror/Comedy mainly due to the many fantastic ways in which people found to close their eyes to what was going on.
This brings me round to an interesting issue with regards to stories based on real events. For example no-one seems in the least offended about tales based around Jack the Ripper although there are obviously people alive who can trace ancestory back to known victims. I can think of several tales of horror/terror based on events from the 50s, 60s or 70s, where it is thought best not to make them too titillating or exploitative.
Obviously an amusement Park based on the Manson Murders is probably still a generation too early, but my point is that there is a loose guideline somewhere inside peoples' heads as to when an horrific event becomes suitable. If anyone has any ideas about how this is worked out I would love to know.
Also examples of films that buck the trend and how they get away with it would be interesting. The young poisoners handbook springs readily to mind.

cheers.