Beverley Allitt, Robert Black, Ian Brady, Myra Hindley, John Childs, Bruce George Peter Lee, Kenneth Erskine, Anthony Hardy, Colin Ireland, Michael Lopo, Patrick Mackay, Robert Mawdsley, Peter Moore, Lenny Murphy, Dennis Nilsen, Mark Rowntree, Harold Shipman, Peter Sutcliffe, Fred West, Rosemary West, Graham Frederick Young.
19 if you count Brady/Hindley and the Wests as one each.
I should have said that those were the serial killers. There have of course been many atrocities that are therefore not in the list, like the killer of the two little girls in Soham.
A serial killer is someone who commits three or more murders over an extended period of time with cooling-off periods in between. In between their crimes, they appear to be quite normal, There is often � but not always � a sexual element to the murders. All the murders must be completed/attempted in a similar fashion or the victims must have something in common, ex. occupation, race, sex, etc.