We're not going to give a bronze medal to a concept.
There is a specific character who can make a firm claim to at least 5 king-killings.
For the record, my final count give Morgoth six kills (three of them by his forces at his direct command), and Sauron ten(!!!) firm kills, plus another three if we count the three Kings of Gondor killed by Easterlings/Wainriders under his presumed command. I'm not giving him credit for kills by random Haradrim, Black Numenoreans, and Orcs - only for cases where he sent an army with a reasonable expectation of killing a specific king.
I count 46 Kings who died in an untimely manner (43 if you discount Maedhros, who wasn't High King when he died). I credit my candidate with 5 kills, Gothmog with 2, and I can only find one other character who claims more than 1 kill (also achieving 2). I originally thought Glaurung qualified, but apparently Azaghal was
Lord of Belegost, so he only gets Orodreth.
hS