Imagine an OCD serial killer, OCD about gravestones. That could totally work.
Stretching that idea out a bit. Every serial killer has an obsession that they have to fulfil by killing people. Well, the obsession of this serial killer is gravestone death dates.
Most are different to the birth date, and that annoys our serial killer every time they go to a cemetery.
14-12-1884 – 28-4-1945.
Dates like that annoy them (and incorrect spelling and grammar on gravestones). Everyone should die on the date they were born, just so the gravestones look better.
Our serial killer is pretty much just like every other serial killer, except victims only have to be nervous on one day of the year. How nice of our serial killer. With most serial killers they make you worry everyday, but this one means you can stop worrying. Well, most of the year.