If OCD about “numbers” (and we all have some of it, otherwise why are people afraid of “Friday the 13th?” - it was unlucky only for the Templars), then it truly might be that he dislikes the number 7. 07-13, or 13-2017. But it might have been that the first killing randomly fell on 07-13, and maybe afterwards he was lucky, and now he is somehow tied up to “doubles” and numbers.
There might be even "triples" as long as "no less than two" I think. And - the idea of "one case fulfilling all the conditions per each state" might be feasible. These connections are hard to trace because what do we know about a person's OCD? A date, with numbers, at least two people, either near water or a flat clearing, or "a meadow next to water"? Could it be the case?
ETA: he might be randomly "closing" the months. 07-13, 02-13, one year it will be 10-13, etc. Probably more likely than disliking nr 7, as I initially thought. So maybe in the database, one could look for killings falling on the 13th of each month, with at least a couple killed, fould close to water on a clearing, and see what else pans out? Some may not be found, as the intent is not for them to be found.