Not an answer but I think it would serve you better to just look up strings of vigilante crimes or states where "mob justice" was the ruling authority.

Since you put serial killer in quotation marks I'm going to assume that you have a loose meaing on it so let me offer another answer. Militaries across the world have trained men that do kill people rather frequently. These people are 99% percent of the time bad people, terrorist, drug dealers, warlords, generally people who have a tendency to rape and kill. I would never label any countries men and women who serve as serial killers but given my interpretation on the question, I feel like they somehow fit your bill.