Well,if you're talking murderers, it's up for debate between Joe Stalin and Mao Tse Tung, who have a combined body count of over 100 million people. Mao got as many as 30-40 mil in 1959 during his great purge.
But as far as serial killers(implied by the thread title) I'd have to go with Carl Panzram, just an overall bastard to the core. He once had one of the longest criminal rapsheets ever.
Here's some goodies about him-
http://en.wikipedia.org/wiki/Carl_Panzram
His last words to the executioner- "Yes, hurry it up, you Hoosier bastard! I could hang 10 men while you're fooling around!"
Now that's going with style. :D