Depends how you define serial killer. If it's defined as any murderer who has killed more than one or two people, then with the gun crime in the USA, it's not surprising it's top. But I suspect that, at least per head of population, Russia, Ukraine and South American countries have produced more real serial killers.
Certainly countries such as Russia, Columbia, and the Ukraine, produce the worst serial killers. Look at this Wikipedia 'top list' of the worst ones by number of victims, and you'll see that Columbia has three in the top ten, and Russia/Ukraine three also. The USA only has one. It's the same further down the list.
https://en.wikipedia.org/wiki/List_o...ber_of_victims