Last year introduced the Puppeteer, who logically serves as Bones’s final serial killer, since he was quite fond of skeletons himself.
Ravi Kapoor’s Dr. Roshan would have killed Zack, too, if it weren’t for those meddling anthropologists.
Which, of course, is when Booth decided to be skeptical about it, and it was revealed in the final minutes that the Puppeteer was actually Zack’s physician, Dr. Roshan.